MEDICAL 


Gift  of 


Lucile   Balaam 


AN   INTRODUCTION  TO  ZOOLOGY 


THE  MACMILLAN  COMPANY 

NEW  YORK  •   BOSTON       CHICAGO 
SAN  FRANCISCO 

MACMILLAN  &  CO.,  LIMITED 

LONDON       BOMBAY  •   CALCUTTA 
MELBOURNE 

THE  MACMILLAN  CO.  OF  CANADA,  LTJX 

TORONTO 


AN   INTRODUCTION  TO 
ZOOLOGY 


BY 


ROBERT  W..HEGNER,  PH.D. 


ASSISTANT  PROFESSOR  OF  ZOOLOGY  IN  THE  UNIVERSITY 
OF  MICHIGAN 


H5 
1110 


If  ctfc 

THE   MACMILLAN    COMPANY 
1924 

rights 


All  rights  reserve  A 


PRINTED   IN   THE   UNITED   STATES   OF  AMERICA 


COPYRIGHT,  1910. 
BY  THE   MACMILLAN   COMPANY. 


Set  up  and  electrotyped.  Published  November,  1910.  Reprinted 
June,  1911;  May,  December,  1912;  November,  1915;  March,  1916; 
September,  1917 ;  February,  October,  1919;  November,  1920;  October, 
November,  1921;  January,  1924. 


Norfcoooti  ^rtss 

J.  S.  Gushing  Co.  —Berwick  &  Smith  Co. 
Norwood,  Mass.,  U.S.A. 


PREFACE 

THIS  book  has  been  written  for  the  use  of  students  taking  the 
introductory  course  in  Zoology  in  Universities  and  Colleges.  It 
has  been  prepared  especially  for  the  zoological  part  of  the  work 
in  General  Biology  at  the  University  of  Michigan,  and  is  ex- 
pected to  supplement  the  one  lecture  and  four  hours  of  labora- 
tory work  per  week  during  the  first  half  year. 

No  textbook  now  on  the  market  covers  the  field  of  the  intro- 
ductory course  in  Zoology  as  it  is  given  at  several  of  the  leading 
Universities.  These  courses  deal  with  invertebrate  types  only, 
being  followed  by  a  course  on  vertebrate  types  during  the  second 
half  year.  Only  a  few  animals  belonging  to  the  more  important 
phyla,  as  viewed  from  an  evolutionary  standpoint,  are  considered. 
They  are,  however,  intensively  studied  in  an  endeavor  to  teach 
the  fundamental  principles  of  Zoology  in  a  way  that  is  not  pos- 
sible when  a  superficial  examination  of  types  from  all  the  phyla 
is  made.  Furthermore,  morphology  is  not  specially  emphasized, 
but  is  coordinated  with  physiology,  ecology,  and  behavior,  and 
serves  to  illustrate  by  a  comparative  study  the  probable  course  of 
evolution.  The  animals  are  not  treated  as  inert  objects  for 
dissection,  but  as  living  organisms  whose  activities  are  of  funda- 
mental importance.  No  arguments,  I  believe,  are  necessary  to 
justify  this  method  of  procedure,  since  the  so-called  "  type 
course,"  developed  with  the  problems  of  organic  evolution  in 
mind,  and  dealing  with  dynamic  as  well  as  static  phenomena,  is 
recognized  by  most  teachers  at  the  present  time  as  the  best 
method  of  introducing  young  students  to  Zoology. 

The  author  has  taught  a  course  similar  to  that  just  outlined  at 

the  University  of  Wisconsin  and  to  three  successive  classes  at  the 

« 

48641 


VJ  PREFACE 

University  of  Michigan,  and  has  based  his  presentation  not  alone 
upon  his  own  experiences,  but  has  profited  by  the  experiences  of 
other  instructors  with  whom  he  worked. 

No  originality  is  claimed  for  this  textbook.  The  material 
contained  in  it  has  been  collected  from  many  sources,  and  an 
attempt  has  been  made  to  incorporate  the  results  of  the  latest 
investigations.  The  majority  of  the  figures  have  been  borrowed 
from  other  textbooks  and  from  original  articles  in  scientific 
periodicals.  The  author  is  indebted  to  the  Macmillan  Com- 
pany for  the  majority  of  these  illustrations.  He  wishes  also, 
to  thank  the  following  for  the  use  of  certain  figures :  Henry 
Holt  and  Co.,  149-156;  New  Era  Printing  Co.,  26,  62,  63; 
A.  I.  Root  Co.,  131,  147;  Science  Press,  91,  109,  no,  in,  116; 
United  States  Department  of  Agriculture,  135,  137,  141, 
142;  Wistar  Institute  of  Anatomy  and  Biology,  12,  13,  25, 
94.  In  every  case  the  author's  name  follows  the  legend  under 
the  figures.  Figures  i,  2,  3,  31,  49-51,  54,  123,  and  157  were 
drawn  by  Mr.  George  M.  Curtis  under  the  author's  direction. 

The  sources  of  information  could  not  well  be  acknowledged 
directly,  so  the  titles  of  the  most  important  and  easily  accessible 
books  and  original  articles  have  been  arranged  at  the  end  of  the 
book  under  the  headings  of  the  various  chapters.  The  titles 
are  numbered  and  are  referred  to  by  number  in  many  places  in 
the  text.  It  is  hoped  that  this  list  will  be  helpful  to  teachers, 
and  that  students  may  be  encouraged  to  supplement  their  own 
observations  and  discoveries  by  consulting  the  original  papers 
dealing  with  the  topics  under  discussion. 

In  a  first  course  in  Zoology,  students  encounter  a  large 
number  of  scientific  terms  which  cause  them  more  or  less 
confusion.  For  this  reason  a  glossary  has  been  included. 
It  gives  the  meaning  of  the  most  important  terms  used,  a 
key  to  their  pronunciation,  and  shows  their  derivation. 

In  making  acknowledgments  it  is  necessary  to  explain  that 
the  present  author  planned  the  book  with  Dr.  A.  S.  Pearse,  and 
that  the  latter  was  to  have  prepared  half  of  the  manuscript 


PREFACE  V1J 

Lack  of  time,  however,  prevented  him  from  completing  his  share 
of  the  work.  Chapters  I,  II,  and  III  were  written  by  Dr.  Pearse; 
Chapters  I  and  II  were  then  revised  jointly,  and  Chapter  III  was 
completed  by  the  present  author  alone.  I  wish  to  express  my 
gratitude  to  Dr.  Pearse  for  allowing  me  to  use  the  first  three 
chapters,  for  reading  most  of  the  remaining  chapters,  and  for 
many  valuable  criticisms  during  the  preparation  of  the  manu- 
script. 

In  an  endeavor  to  make  the  text  as  nearly  correct  as  pos- 
sible, the  author  sent  the  various  chapters  to  specialists  on 
the  subject-matter  treated  therein.  These  gentlemen,  how- 
ever, are  in  no  way  responsible  for  any  errors  that  may  have 
escaped  notice.  The  author  is  grateful  to  Professor  Jacob 
Reighard  for  criticisms  during  the  preparation  of  the  work 
and  for  reading  Chapter  XIV,  to  Professor  S.  J.  Holmes  for 
reading  the  entire  manuscript,  to  Rev.  H.  F.  Hegner  for  read- 
ing Chapters  I-VIII,  to  Professor  C.  M.  Child  for  reading 
Chapters  I-III,  to  Professor  F.  G.  Novy  and  Dr.  H.  Hus  for 
reading  Chapters  IV-VI,  to  Dr.  O.  P.  Dellinger  for  reading 
Chapter  IV,  to  Professor  L.  L.  Woodruff  for  reading  Chapter 
V,  to  Professor  T.  H.  Montgomery,  Jr.,  for  reading  Chap- 
ter VII,  to  Dr.  E.  R.  Downing  for  reading  Chapter  VIII,  to 
Professor  W.  C.  Curtis  for  reading  Chapter  IX,  to  Professor 
J.  Percy  Moore  for  reading  Chapter  X,  to  Dr.  A.  E.  Ortmann 
for  reading  Chapter  XI,  to  Dr.  E.  F.  Phillips  and  Mr.  Max 
Peet  for  reading  Chapter  XII,  and  to  Dr.  A.  G.  Ruthven  and 
Dr.  H.  Hus  for  reading  Chapter  XIV. 

ROBERT  W.  HEGNER. 


TABLE  OF   CONTENTS 

CHAPTER  I 

PAGE 

INTRODUCTION     .,...«....        I 

Definitions,  I ;  Classification,  3. 

CHAPTER  II 

PHENOMENA  OF  LIFE.        „  8 

Origin  of  life,  8 ;  Characteristics  of  living  organisms,  10 ; 
Plants  and  animals  compared,  17  ;  Physical  basis  of  life :  pro- 
toplasm, 19 ;  Physico-chemical  explanation  of  the  phenomena 
of  life,  23, 

CHAPTER  III 

CELL  AND  CELL  THEORY  . 26 

Morphology  and  physiology  of  the  cell,  26;  Cell  division. 
29 ;  Cell  theory,  34. 

CHAPTER  IV 
AMEBA       .       »       .    .    ,.        ,       .       «.       .       ,       .       -37 

CHAPTER  V 
PARAMECIUM      .....««..•     Jp 

CHAPTER  VI 
OTHER  PROTOZOA 80 

Classification  of  the  Protozoa,  80 ;  Euglena,  82 ;  Plasmo 
dium,  88 ;  Volvox  globator  and  its  allies,  92. 

CHAPTER  VII 

INTRODUCTION  TO  THE  METAZOA 100 

Cellular  differentiation:  Tissues,  100;  Germ  cells,  102 ; 
Embryology,  107 

fc 


X  TABLE  OF  CONTENTS 

CHAPTER  VIII 

PAGE 

HYDRA  AND  CCELENTERATES  IN  GENERAL        .        .        .        .116 
Hydra,  116;  Coelenterates  in  general,  139. 

CHAPTER   IX 
SPONGES,  FIAT  WORMS,  AND  ROUND  WORMS  ....     144 

Sponges —  Grantia —  Sponges  in  general,  144  ;  Flat  Worms 
—  Planaria—  Flat  Worms  in  general,  153  ;  Round  Worms  — 
Ascaris  —  Round  Worms  in  general,  160. 

CHAPTER   X 

THE  EARTHWORM  AND  ANNELIDS  IN  GENERAL         .        .        .     164 
The -earthworm,  164;  Annelids  in  general,  190. 

CHAPTER  XI 

THE  CRAYFISH  AND  ARTHROPODS  IN  GENERAL        .        „        .     193 
The  crayfish,  193;  Arthropods  in  general,  225. 

CHAPTER  XII 

THE  HONEYBEE  AND  BEES  IN  GENERAL 233 

The  honeybee,  233  ;  Bees  in  general,  260. 

CHAPTER  XIII 
HISTORICAL  ZOOLOGY         ........    267 

CHAPTER  XIV 
GENERAL  CONSIDERATIONS  OF  ZOOLOGICAL  FACTS  AND  THEORIES    275 

Heredity  and  evolution,  275  ;  The  social  life  of  animals, 
297 ;  The  animal  mind,  305. 

BIBLIOGRAPHY 307 

GLOSSARY 323 

INDEX  0    337 


LIST  OF   ILLUSTRATIONS 

FACING  PAGE 

Parts  of  Paramecia  showing  cilia  and  trichocysts  ...  62 
A  section  from  the  region  of  the  mouth  of  Paramecium  showing 

the  formation  of  a  food  vacuole 66 

Binary  fission  of  Paramecium  aurelia 66 

Diagram  illustrating  the  life  history  of  the  malarial  fever  parasite  89 
Volvox  globator  .  .  ...  .  .  .  .  between  96-97 

Volvox  globator between  96-97 

Examples  of  various  kinds  of  tissues 100 

Figures  illustrating  four  different  kinds  of  cleavage  .  .  .  109 

Regeneration  and  grafting  in  Hydra 137 

Physalia,  the  "  Portuguese  man-of-war " 143 

Regeneration  of  Planar ia  maculata 157 

Longitudinal  vertical  section  through  the  anterior  portion  of  an 

earthworm 168 

Stages  in  the  embryology  of  the  earthworm 185 

Regeneration  in  the  earthworm 189 

Grafting  in  the  earthworm    ...                  ....  190 

Semi-diagrammatic  view  of  internal  organs  and  appendages  of 

right  side  of  a  male  crayfish 195 

Surface  of  a  crayfish  egg  with  embryo  beginning  to  form  .  .216 
Embryo  of  the  crayfish  in  the  Nauplius  stage  .  .  .  .216 

Older  embryo  of  the  crayfish 216 

Head  of  worker  honeybee 235 

Legs  of  worker  bee 237 

Sting  of  the  worker  bee 239 

The  respiratory  system  of  the  bee 243 

The  nervous  system  of  the  bee 244 

Female  reproductive  organs 249 

ni 


«ll  LIST  OF  ILLUSTRATIONS 

FACING  PAGE 

Larvae  and  pupa  of  honeybee  in  their  cells 253 

Internal  organs  of  larval  honeybee 253 

Pollination  of  an  orchid  (Cypripedimri)  by  a  bumblebee       .        ,  263 

Aristotle,  384-322  B.C 267 

Linnaeus  at  sixty,  1707-1778 269 

Cuvier,  1769-1832 27O 

William  Harvey,  1578-1667 271 

Karl  Ernst  von  Baer,  1792-1876 272 

Johannes  Mtiller,  1801-1858 273 

Charles  Darwin,  1809-1882 274 

Varieties  of  domestic  pigeons *  294 


AN   INTRODUCTION   TO  ZOOLOGY 


AN  INTRODUCTION  TO  ZOOLOGY 


CHAPTER  I 

INTRODUCTION 

i.  DEFINITIONS 

THE  science  of  Biology  includes  the  sum  of  human  knowledge 
with  regard  to  organisms.  That  branch  of  biology  which  deals 
with  animals  is  known  as  Zoology;  that  which  relates  to  plants, 
as  Botany.  Zoology  may  be  defined  as  the  body  of  facts  and 
doctrines  derived  from  the  scientific  study  of  animals.  Scien- 
tific study  demands  accurate  and  painstaking  observation  carried 
on  with  some  definite  end  in  view.  The  scientific  study  of 
zoology  attempts  to  gain  an  understanding  of  the  structure  and 
activities  of  animals.  It  not  only  deals  with  the  animals  per  se, 
but  also  with  their  relations  to  both  the  organic  and  inorganic 
worlds. 

The  detailed  investigation  of  animals  has  led  to  the  establish- 
ment of  a  number  of  subsidiary  zoological  sciences,  several  of 
which  are  briefly  outlined  in  the  next  few  paragraphs. 

Systematic  Zoology  is  concerned  with  the  description  of  animal 
species  and  their  arrangement  according  to  a  logical  plan  of 
classification.  The  exact  meaning  of  the  term  species  is  a 
live  question  at  the  present  time,  and  systematic  zoologists  do 
not  agree  as  to  what  characteristics  should  be  used  in  separating 
one  species  from  another.  One  investigator  in  this  field  gives 
the  following  definition:  "A  species,  as  conceived  by  most 
systematists  at  the  present  time,  may  be  defined  as  a  group  of 
interbreeding  individuals  which,  while  they  may  differ  markedly 
among  themselves,  yet  resemble  each  other  more  closely  than  they 
do  those  of  any  other  group;  the  characters  that  distinguish  the 


2  AN  INTRODUCTION  TO  ZOOLOGY 

group  being  considerable,  not  obliterated  by  intermediate  forms, 
and  inherited  from  generation  to  generation."  Another  promi- 
nent zoologist,  when  asked  for  a  definition  of  a  species,  said  it 
was  "  somebody's  opinion."  He  did  not  mean  that  species  are 
not  realities  in  nature,  but  that  just  how  many  are  represented 
in  a  particular  group  must  usually  be  determined  by  competent 
authority. 

Distributional  Zoology  attempts  to  ascertain  the  past  and 
present  habitats  of  animals,  as  well  as  the  factors  which  control 
their  distribution  over  the  surface  of  the  earth.  The  study  of  the 
distribution  of  recent  animals  is  known  as  Zoogeography,  and  that 
of  fossil  forms  constitutes  a  part  of  the  science  of  Paleontology. 

Animal  Morphology  is  the  science  of  form  and  structure.  That 
part  of  it  which  pertains  to  the  gross  dissection  of  organs  is  known 
as  Anatomy;  that  which  relates  to  the  microscopic  study  of 
tissues,  as  Histology.  If,  however,  a  study  is  concerned  with  the 
development  of  animals,  organs,  or  tissues,  it  is  included  in  the 
science  of  Embryology. 

Animal  Physiology  deals  with  the  functions  of  the  parts  of 
animals. 

Animal  Ecology  is  the  study  of  the  relationships  of  animals 
to  one  another  and  to  their  environment. 

Evolutionary  Zoology  is  concerned  with  the  origin  and  descent 
of  the  different  species  of  animals. 

Paleozoology  is  the  study  of  fossil  remains  of  animals.  It  is 
intimately  associated  with  all  the  other  branches  of  zoological 
science,  except  the  study  of  physiological  processes.  The  physi- 
ology of  extinct  forms  can  only  be  inferred  from  the  study  of 
the  activities  of  recent  animals.  Owing  to  the  fact  that  fossils 
are  usually  embedded  in  stone,  the  mechanical  appliances  which 
are  used  in  studying  them  are  necessarily  quite  different  from 
those  employed  in  the  investigation  of  modern  animals. 

The  different  branches  of  zoological  science  are  closely  related 
and  mutually  interdependent;  for  example,  the  students  of  evo- 
lution gain  evidence  concerning  the  ancestry  of  animals  from 


INTRODUCTION  3 

all  the  other  fields  of  zoological  investigation.  From  a  careful 
examination  of  fossil  forms,  it  is  believed  that  the  horse  originated 
from  a  five-toed  ancestor.  This  theory  can  be  supported  by 
evidence  gained  from  the  study  of  the  embryology  of  modern 
horses.  The  physiology  and  ecology  of  horses  give  further 
evidence  as  to  why  some  of  the  horse-like  animals  persisted  to 
the  present  time  while  others  became  extinct,  and  why  horses 
are  now  found  in  some  regions  of  the  earth  and  not  in  others. 

2.   CLASSIFICATION 

Animals  are  not  infinitely  variable.  Their  investigation  has 
resulted  in  the  establishment  of  about  five  hundred  thousand 
species.  These  are  grouped,  according  to  similarity  in  structure, 
into  about  fifteen  large  divisions,  or  Phyla.  One  of  the  chief 
aims  of  zoologists  in  the  past  has  been  to  give  a  complete  descrip- 
tive inventory  of  the  animal  kingdom.  The  large  number  of 
species  which  have  been  described  has  made  it  necessary  to  sepa- 
rate each  of  the  phyla  into  smaller  and  smaller  subdivisions. 
Those  animals  which  are  most  nearly  related  have  therefore  been 
placed  together,  and  certain  definite  names  are  now  generally 
adopted  for  use  in  classification.  Thus  under  each  phylum  one 
or  more  classes  are  included,  and  under  each  class,  a  variable 
number  of  orders.  In  the  following  list  of  such  terms  the 
groups  become  successively  smaller:  phylum,  class,  order,  family, 
genus,  species,  individual. 

An  example  will  perhaps  make  the  system  more  clear.  George 
Washington  was  an  individual;  he  belonged,  with  other  men, 
to  the  species  sapiens  of  the  genus  Homo.  This  genus,  together 
with  another  of  somewhat  questionable  relationships,  the  extinct 
Pithecanthropus,  constitutes  the  family  Hominida.  The  Homi- 
nid(E  are  included  with  ten  other  families  of  monkey-like  animals 
in  the  order  Primates.  Fifteen  related  orders,  of  which  the  Pri- 
mates form  one,  are  placed  in  the  class  Mammalia.  All  mammals 
possess  hair  and  mammary  glands;  these  characteristics  dis- 


AN  INTRODUCTION  TO  ZOOLOGY 


tinguish  them  from  the  four  other  classes  which  make  up  the 
phylum  Vertebrata,  or  animals  possessing  vertebral  columns. 
The  scientific  name  of  any  animal  consists  of  the  terms  used  to 
designate  the  genus  and  species;  this  is  commonly  followed  by 
the  name  of  the  zoologist  who  wrote  the  first  authoritative  de- 
scription of  that  particular  species.  The  scientific  name  of  man 
is  therefore  written  Homo  sapiens  Linnaeus. 

The  following  concise  synopsis  of  the  chief  groups  of  animals, 
together  with  a  brief  characterization  of  each  of  the  fifteen  phyla, 
will  be  of  value  to  the  student  for  subsequent  reference.  The 
terms  used  in  describing  these  subdivisions  will  not  perhaps  be 
altogether  understood  by  the  beginner,  but  their  meaning  will 
be  made  apparent  in  succeeding  chapters.  It  will  suffice  to 
say  here  that,  in  a  general  way,  the  classification  of  animals 
depends  chiefly  upon  the  characteristics  which  are  contrasted 
in  Table  I. 

TABLE  I 

CHIEF  CHARACTERISTICS   USED  IN   SEPARATING  THE   PHYLA  OF  ANIMALS 
CONTRASTED 


1.  Body  composed  of  one  microscopic 

unit,  the  cell. 

2.  Body  formed  from    two  primary 

cell  layers,  i.e.  diploblastic. 

3.  Body    radially     symmetrical,    i.e. 

parts  (antimeres)  radiating  from 
a  central  axis. 

4.  Digestive    cavity    with   only   one 

opening. 

5.  A  cavity,  the  ccelom  between  the 

digestive  tube  and  the  body  wall. 

6.  Body  segmented,  i.e.  composed  of 

a  lineal  series  of   parts,    called 
metameres  or  somites. 


Body  composed  of  many  cells. 

Body  formed  from  three  primary  cell 

layers,  i.e.  triploblastic. 
Body    bilaterally  symmetrical,     i.e. 

parts  arranged  in  pairs  on  either 

side  of  a  central  plane. 
Digestive  cavity  with  two  openings, 

the  mouth  and  anus. 
No  ccelom  present. 

Body  unsegmented. 


The  figures  following  the  diagnosis  of  each  phylum  indicate 
the  number  of  recent  species;  those  in  parentheses,  the  number  of 
fossil  species  known  to  represent  it. 


INTR©BUCTION  5 

PRINCIPAL  PHYLA  OF  ANIMAL  KINGDOM1 

1.  Protozoa.  —  Single-celled  animals;    often  colonial;    sperm 
and  egg  cells  usually  wanting.     4400  (2000). 

2.  Porifera.  —  Sponges.     Triploblastic  (?) ;  radially  symmetri- 
cal, number  of  antimeres  variable;  body  wall  permeated  by 
innumerable  pores  and  usually  supported  by  a  skeleton  of  spicules. 
600  (800). 

3.  Coelenterata.  —  Jellyfishes,    polyps,    and     corals.     Diplo- 
blastic;  radially  symmetrical,  with  four  or  six  antimeres;  single 
gastro- vascular  cavity;    no  anus;  body  wall  contains  peculiar 
bodies  known  as  nematocysts  or  stinging  cells.     3000  (1780). 

4.  Ctenophora.  —  Sea  walnuts  or  comb  jellies.     Triploblastic; 
radial  combined  with  bilateral  symmetry;  eight  radially  arranged 
rows  of  paddle  plates;  anus  present.     1000  (no  fossils). 

5.  Platyhelminthes. — Flat  worms.     Triploblastic;  bilaterally 
symmetrical;   single  gastro- vascular  cavity;   no  anus;  presence 
of  coelom  doubtful.     1600  (no  fossils). 

6.  Nemathelminthes.  —  Round    worms.     Triploblastic;     bi- 
laterally symmetrical ;  possess  a  tubular  digestive  system  with  an 
anus;  ccelom  present.     1000  (no  fossils). 

7.  Rotatoria. — Wheel  animalcules.     Triploblastic;  bilaterally 
symmetrical;  coelom  present;  a  pair  of  ciliated  disks  on  the  an- 
terior end.     350  (no  fossils). 

8.  Bryozoa.  —  Moss  animals.     Triploblastic;    bilateral  sym- 

1  Zoologists  do  not  agree  as  to  the  number  of  phyla  into  which  the  animal 
kingdom  should  be  divided.  Some  authorities  recognize  only  eight,  while 
others  maintain  that  there  should  be  as  many  as  twenty,  or  even  more.  Two 
sub-kingdoms  are  generally  recognized,  Protozoa  (Phylum  i)  and  Melazoa 
(Phyla  2-15).  Recently  many  zoologists  have  come  to  believe  that  the 
sponges  (Phylum  2)  should  be  separated  from  other  metazoons  and  called 
the  Parazoa.  The  animals  in  Phyla  1-14  are  usually  called  invertebrates  to 
distinguish  them  from  the  vertebrates  (15).  Phyla  14  and  15,  with  a  few  ani- 
mals of  somewhat  doubtful  relationships,  are  commonly  placed  together 
under  the  group  Chordata,  and  are  known  as  chordates.  Phyla  2-4  include 
animals  without  coeloms  and  are  known  as  Accelomata  in  contrast  to  phyla 
5-15,  which  contain  animals  with  a  coelom  and  are  termed  Calomata. 


6  AN  INTRODUCTION  TO  ZOOLOGY 

metry  usually  distorted  by  colonial  habits;    ciliated  tentacles 
about  the  mouth;  coelom  and  anus  present.     700  (1800). 

9.  Brachiopoda.  —  Tongue    or    lamp    shells.     Triploblastic; 
bilaterally  symmetrical;  ccelom  present;  anus  present  or  absent; 
ciliated  arms  around  the  mouth;   body  covered  by  a  shell  com- 
posed of  dorsal  and  ventral  valves.     100  (2500). 

10.  Echinodermata.  —  Starfishes,  sea  cucumbers,  sea  urchins, 
sea    lilies.     Triploblastic;     radially    symmetrical,    usually    five 
antimeres;   coelom  well  developed;   anus  usually  present;   loco- 
motion in  many  species  accomplished  by  characteristic  organs 
known  as  tube  feet;  a  spiny  skeleton  of  calcareous  plates  gener- 
ally covers  the  body.     2500  (2600). 

11.  Annelida.  —  Jointed    worms.     Triploblastic;     bilaterally 
symmetrical;   coelom  well  developed;   anus  present;  segmented, 
somites  similar.     2500  (25). 

12.  Arthropoda.  —  Crabs,   insects,   spiders,   centipedes,   scor- 
pions,   ticks.      Triploblastic;    bilaterally    symmetrical;     anus 
present;    coelom  poorly  developed;   segmented,  somites  usually 
more  or  less  dissimilar;  paired  jointed  appendages  present  on  all 
or  a  part  of  the  somites;  chitinous  exoskeleton.    200,000  (5000). 

13.  Mollusca.  —  Clams,    snails,     devilfishes.      Triploblastic; 
bilaterally  symmetrical;  anus  and  coelom  present;  no  segmenta- 
tion; shell  usually  present;  the  characteristic  organ  is  a  ventral 
muscular  foot.     22,000  (21,000). 

14.  Tunicate.  —  Sea  squirts.     Triploblastic;  bilaterally  sym- 
metrical;   coelom  and  anus  present;  form  saccular  or  barrel- 
shaped;  two  apertures  through  which  water  currents  pass  in  and 
out;   frequently  colonial;   all  pass  through  a  tadpole-like  larval 
stage  during  which  the  body  is  supported  by  a  rod-like  structure, 
the  chorda.     300  (no  fossils). 

15.  Vertebrata.  —Bilaterally   symmetrical;    coelom   well   de- 
veloped; anus  present;  chorda  usually  more  or  less  fully  replaced 
by  a  series  of  bony  vertebrae;   many  systems  of  internal  organs 
segmented;  two  pairs  of  appendages  usually  present;    central 
nervous  system  always  dorsal  to  digestive  tube.     25,000  (2400). 


INTRODUCTION 


FIG.  i. 


Figure  i  is  intended  to  show  the  relationships  of  the  different 
phyla  of  the  animal  kingdom  to  each  other.  The  Protozoa  are 
believed  to  have  given  rise  to  the  other  groups.  The  vertical 
height  of  each  phylum  upon  the  page  represents  the  degree  of 
structural  specialization  which  it  has  attained. 


CHAPTER  II 
PHENOMENA  OF  LIFE 

i.  ORIGIN  OF  LIFE 

THOSE  who  are  inclined  toward  the  study  of  living  organisms 
will  find  an  abundance  of  material  on  every  hand.  Darwin 
raised  five  hundred  and  thirty-seven  plants  from  three  spoon- 
fuls of  mud  which  he  scooped  up  at  the  edge  of  a  pond.  A  recent 
naturalist  has  collected  from  four  square  feet  of  meadow  over  a 
thousand  objects  representing  living  animals  or  their  remains. 
If  we  go  to  the  depths  of  the  sea,  or  to  the  tops  of  the  mountains, 
or  even  penetrate  the  darkness  of  the  caverns  beneath  the  earth, 
we  always  find  forms  of  animal  life  which  have  become  adapted 
to  almost  every  conceivable  condition  of  existence. 

The  great  abundance  and  rapid  multiplication  of  living  animals 
led  the  ancients  to  formulate  some  curious  theories  with  regard 
to  the  origin  of  life.  Even  at  this  enlightened  day  it  may  be 
helpful  to  us  to  review  some  of  their  peculiar  ideas.  Many  of 
the  scholars  of  that  period  believed  that  every  animal  was  origi- 
nally created  by  divine  providence.  A  favorite  theory  with 
those  who  did  not  accept  this  idea  of  special  creations,  was  that  of 
spontaneous  generation.  This  was  a  universally  accepted  dogma 
from  the  time  of  the  Greek  philosopher  Aristotle  (384-322  B.C.) 
until  1668  when  Redi  overthrew  it  with  his  careful  observations. 

Ancient  naturalists  believed  that  frogs  and  toads  arose  from  the 
muddy  bottom  of  ponds  under  the  influence  of  the  sun,  and  that 
insects  originated  from  dew.  Those  who  did  not  accept  these 
and  other  equally  absurd  views  were  subject  to  ridicule  by  their 
contemporaries.  Before  Redi  performed  his  simple  but  remark- 
ably effective  experiments,  no  one  had  thought  of  testing  the 

8 


PHENOMENA  OF  LIFE  9 

verity  of  the  theory  of  spontaneous  generation.  Redi  proved  con- 
clusively that  maggots  never  rise  de  now  from  decaying  flesh,  as 
had  been  believed  up  to  his  time.  He  exposed  meat  in  three  jars 
under  diverse  conditions;  one  was  left  open,  the  second  was 
covered  with  gauze,  and  the  third  was  covered  with  parchment. 
The  meat  decayed  in  all  of  the  jars,  but  maggots  appeared  only 
in  the  meat  in  the  one  which  had  been  left  entirely  uncovered. 
Flies  were  attracted  by  the  decaying  meat  in  the  second  jar,  but, 
being  unable  to  enter,  laid  their  eggs  on  the  gauze,  where  they 
were  found  by  Redi.  No  eggs  were  laid  on  the  third  jar,  for  the 
parchment  did  not  allow  odors  to  escape.  By  these  and  other 
experiments  the  theory  of  spontaneous  generation  fell  into  dis- 
repute, but  many  of  its  adherents  still  maintained  that  even 
though  some  animals  might  not  originate  spontaneously,  that 
fact  did  not  preclude  the  possibility  of  others  doing  so. 

Such  dissenters  made  their  last  stand  when  the  microscope 
revealed  the  swarms  of  minute  animals  which  had  been  previously 
unknown.  In  one  of  their  experiments  hay  was  boiled  in  water 
until  all  the  life  had  been  killed;  the  liquid  was  then  filtered  off 
and  placed  in  a  tightly  stoppered  bottle.  The  subsequent 
appearance  of  a  scum  of  living  organisms  in  such  infusions  con- 
vinced them  that  life  had  arisen  spontaneously.  In  1775, 
Spallanzani  proved  that  when  proper  precautions  were  observed 
no  life  developed  in  a  culture  of  this  kind.  He  prevented  the 
entrance  of  air  into  the  vessels  containing  the  nutrient  solution 
by  hermetically  sealing  the  slender  necks  of  his  flasks  in  flame. 
The  flasks  were  then  placed  in  boiling  water  for  three  quarters 
of  an  hour,  all  germs  contained  in  them  being  destroyed.  No 
life  appeared  in  nutrient  fluids  thus  treated.  Despite  this 
demonstration  the  question  continued  to  be  agitated  for  many 
years,  and  it  was  not  until  the  time  of  Pasteur  (1864)  and  Tyndall 
(1876)  that  the  old  theory  of  spontaneous  generation  was  com- 
pletely overthrown. 

At  the  present  time  we  are  able  to  affirm  that  no  living  organ- 
ism is  known  to  originate  except  from  some  other  preexisting 


10  AN  INTRODUCTION  TO  ZOOLOGY 

organism,  and  Prayer's  aphorism  "  all  life  from  life  "  is  univer- 
sally accepted.  But  though  in  the  light  of  our  present  knowledge 
all  are  willing  to  admit  the  truth  of  this  aphorism,  there  are  those 
who  hope  that  the  advance  of  scientific  knowledge  may  some  time 
in  the  future  enable  us  to  clear  up  the  problem  of  the  beginnings 
of  life  which  still  remains  unsolved. 

2.   CHARACTERISTICS  OF  LIVING  ORGANISMS 

All  the  objects  that  come  within  the  range  of  human  experi- 
ence may  be  grouped  into  two  classes,  living  and  non-living. 
Although  our  ideas  concerning  the  origin  of  living  organisms 
cannot  go  beyond  the  assertion  that  they  always  come  from  other 
living  bodies,  yet  the  characteristics  that  separate  the  two  classes 
can  be  stated  in  a  very  definite  way.  Living  organisms  are 
characterized  by  (i)  definite  size,  (2)  definite  form,  (3)  definite 
elementary  composition,  (4)  definite  organization,  (5)  growth  by 
intussusception,  (6)  reproduction,  and  (7)  irritability. 

(i)  Size.  —  If  one  were  asked  the  question,  Are  organisms 
uniform  in  size  ?  he  would  doubtless  answer,  No  !  but  on  more 
careful  consideration  it  would  become  apparent  that,  although  a 
wide  range  of  variation  exists,  yet  the  differences  in  size  are  con- 
fined within  rather  definite  limits.  Certain  parasites  found  in 
human  blood,  which  can  only  be  seen  with  the  highest  powers  of 
the  microscope,  represent  the  smallest  animals  known ;  the  whale 
is  the  largest.  Although  the  discrepancy  between  these  animals 
is  enormous,  yet  the  range  of  variation  illustrated  by  them  has  a 
finite  limit.  On  the  other  hand,  the  size  variations  of  inorganic 
substances  are  infinite ;  for  example,  water  is  always  recognizable 
as  such  whether  it  exists  in  the  form  of  minute  particles  of  vapor 
or  reaches  the  dimensions  of  the  Pacific  Ocean.  Although  a 
wide  range  of  variation  occurs  among  animals  in  general,  the  indi- 
viduals of  any  given  species  are  practically  equal  in  size.  In 
many  instances  it  is  obvious  that  the  size  of  a  certain  species 
fits  it  for  the  conditions  under  which  it  lives.  The  whale  could 
not  change  places  with  one  of  the  micro-organisms  found  in  de- 


PHENOMENA  OF  LIFE 


II 


caying  matter.     In  a  general  way  we  may  say  that  the  structure 
and  habitat  of  any  animal  limits  its  size. 

(2)  Form.  —  The  various  species  of  animals  have  character- 
istic forms  by  which  they  may  as  a  rule  be  recognized.     Many 
organisms  are  extremely  irregular  in  form;  for  example,  sponges 
incrust  the  surfaces  of  rocks  or  piles  in  an  infinite  variety  of 
shapes,  and  no  two  trees  of  the  same  kind  have  a  similar  set  of 
branches,  yet  every  species  has  certain  distinct  peculiarities.     A 
person  who  is  familiar  with  sponges  is  able  to  recognize  the  differ- 
ent species  at  a  glance,  and  any  one  can  distinguish  an  oak  from  a 
poplar  as  far  as  he  can  see  them. 

Non-living  bodies  in  most  cases  have  no  regularity  of  shape.  A 
piece  of  granite  may  form  a  slab  or  a  boulder  or  assume  any  other 
contour;  the  water  in  a  lake  conforms  to  the  shape  of  the  lake-bed. 

(3)  Chemical  Composition.  —  If  a  sufficient  variety  of  inor- 
ganic bodies  are  collected  and  analyzed,  every  one  of  the  eighty 
or  more  chemical  elements  known  to  science  will  be  represented. 
If,  on  the  other  hand,  any  number  of  living  organisms  are  col- 
lected, generally  not  more  than  twelve  elements  can  be  secured 
by  analysis.      Furthermore,  only  five  of  these  will  usually  be 
represented  in  considerable  quantities.     Thus  a  typical  animal 
would  be  found  to  contain  the  following  elements:  — 


Carbon 

Oxygen 

Nitrogen 

Hydrogen 

Sulphur 

Phosphorus 

Chlorine 

Potassium 

Sodium 

Magnesium 

Calcium 

Iron 


99  per  cent  of  weight ; 


i  per  cent  of  weight. 


12  AN  INTRODUCTION  TO  ZOOLOGY 

The  elements  constituting  living  substance,  form  complex 
organic  compounds  which  exhibit  three  striking  peculiarities: 
(i)  they  are  in  most  cases  labile  substances,  i.e.  they  are  un- 
stable and  maybe  broken  up  into  simple  compounds,  or  synthe- 
sized into  more  complex  molecules,  (2)  carbon  is  always  an  im- 
portant constituent,  and  (3)  their  important  elements  are  all 
non-metallic. 

The  chemical  composition  of  living  matter  is  highly  variable. 
A  complete  analysis  of  representatives  of  any  two  species  would 
never  give  identical  results;  and  even  young  and  old  individuals 
of  the  same  species  would  not  agree.  Nevertheless,  all  animals 
show  a  remarkable  uniformity  in  chemical  composition.  Some 
animals  might  be  considered  exceptions  to  this  general  rule; 
for  example,  a  clam  upon  analysis  would  show  a  large  percentage 
of  calcium  carbonate  owing  to  the  presence  of  that  salt  in  its 
shell.  The  shell,  however,  is  really  not  a  part  of  the  living  sub- 
stance, but  represents  an  inorganic  accretion  that  has  formed 
around  the  body. 

(4)  Organization.  —  Although  living  organisms  exhibit  great 
diversity  in  structure,  a  careful  examination  of  all  the  parts 
proves  that  the  fundamental  elements  are  essentially  the  same 
in  every  individual.  All  plants  and  animals  are  composed  of 
similar  microscopic  units,  called  cells,  which  are  arranged  in  a 
characteristic  manner  for  each  species.  Thus  the  body  of  a  very 
simple  organism  may  consist  of  only  a  single  cell,  whereas  that 
of  a  complex  organism  may  contain  billions.  Cells  may  be 
compared  with  bricks  which  are  used  to  build  houses.  Bricks 
may  show  variations  in  form,  color,  and  composition,  but  have 
certain  characteristics  which  make  them  easily  recognizable 
whether  they  are  separate  or  built  into  walls.  In  a  similar  way, 
though  individual  cells  show  minor  differences,  they  agree  in 
fundamental  characteristics,  and  may  readily  be  identified,  even 
when  constituents  of  a  many-celled  animal. 

Whether  an  organism  is  unicellular  or  multicellular,  it  always 
consists  of  a  number  of  unlike  parts  which  are  incapable  of  inde- 


PHENOMENA  OF  LIFE  13 

pendent  existence.  When  the  essential  parts  of  a  living  animal 
are  separated  from  one  another,  they  soon  lose  their  identity 
and  ultimately  disintegrate  into  simple  inorganic  compounds. 
Non-living  bodies  have  no  common  unit  of  structure  which  is 
comparable  to  the  cell,  and  their  parts  show  no  interdependence. 

(5)  Growth  in  living  organisms  is  usually  by  intussusception, 
i.e.  by  the  addition  of  new  particles  among  those  already  present; 
this  results  in  a  swelling  of  every  part,  and  does  not  necessitate 
a  change  in  form.  Non-living  bodies  usually  grow  by  accretion, 
i.e.  the  addition  of  successive  layers  on  the  outside. 

Growth  in  any  living  thing  involves  a  complex  series  of  changes. 
The  chemical  compounds  which  make  up  the  bodies  of  animals 
are  extremely  labile;  they  are  constantly  breaking  down  into 
simpler  substances  or  becoming  more  complex  by  the  addition 
of  new  materials.  There  is  no  time  during  the  life  of  any  indi- 
vidual when  elaborate  chemical  reactions  are  not  taking  place. 
Metabolism  is  the  term  used  to  include  this  great  complex  of 
incessant  changes.  Those  processes  which  use  energy  to  build 
up  compounds  are  said  to  be  anabolic;  those  which  destroy  sub- 
stance to  produce  energy  are  termed  katabolic. 

Animals  are  primarily  katabolic  organisms.  They  cannot 
make  organic  compounds  from  simple  inorganic  substances;  in 
this  respect  they  differ  from  plants  which  manufacture  starch 
from  carbon  dioxide  and  water.  Since  animals  must  have  organic 
food,  it  follows  that  plant  products  are  necessary.  Before  animal 
growth  is  possible,  food  must  be  converted  into  living  substance. 
By  the  process  of  digestion  food  is  prepared  for  absorption.  Some 
substances  that  cannot  be  digested  are  passed  out  of  the  body  as 
faces.  After  absorption,  food  is  carried  to  some  part  of  the  body 
where  it  is  needed;  here  it  is  transformed  into  living  substance 
by  the  process  of  assimilation. 

In  order  that  metabolic  activity  may  go  on  without  ceasing,  a 
constant  supply  of  energy  is  necessary.  This  energy  is  in  part 
furnished  by  oxidation,  i.e.  the  chemical  union  of  oxygen  with  the 
living  substance  in  a  manner  which  may  be  compared  to  the 


14  AN  INTRODUCTION  TO  ZOOLOGY 

changes  taking  place  during  combustion.  Respiration  supplies 
the  oxygen  for  such  metabolic  activities,  and  also  eliminates  cer- 
tain gaseous  excretory  products.  The  waste  products  formed 
by  the  breaking  down  of  living  substance  are  cast  off  as  excretions. 
These  should  not  be  confused  with  faeces,  which  have  never  actu- 
ally become  constituents  of  living  matter. 

Figure  2  is  intended  to  indicate  the  relations  of  the  various 
metabolic  activities.     It  shows  that  an  animal  requires  food  and 


FIG.  2.  Diagram  showing  the  relations  of  the  various  metabolic  activities 

of  animals. 

oxygen;  that  part  of  the  food  is  digested  and  assimilated,  while 
the  rest  is  cast  out  as  faeces;  and  that  the  oxidation  of  the  living 
substance  results  in  the  production  of  certain  excretions  which 
are  eliminated.  Urea  is  the  most  important  of  these  excretory 
products. 

Growth  has  been  said  to  represent  the  excess  of  anabolism 
over  katabolism.  The  power  of  growth  becomes  gradually  less 
throughout  the  life  of  any  individual,  and  all  animals  pass 
through  certain  well-marked  stages,  youth,  maturity,  and  old  age. 
Youth  is  generally  characterized  by  great  vigor  and  activity  as 
well  as  rapid  growth.  At  maturity  anabolism  and  katabolism 
are  about  equally  balanced.  During  this  adult  period  little 
change  is  observable  in  external  form  or  gross  weight.  Maturity 
gradually  gives  way  to  old  age,  a  period  of  decline,  when  the  body 


PHENOMENA  OF  LIFE  15 

slowly  wastes  away  and  the  vital  processes  finally  cease  altogether. 
These  three  periods,  youth,  maturity,  and  old  age  make  up  the 
life  cycle  of  every  individual. 

(6)  Reproduction.  —  Perhaps  the  most  remarkable  charac- 
teristic of  a  living  organism  is  its  ability  to  produce  new  indi- 
viduals like  itself  at  some  time  in  the  life  cycle.  This  reproductive 
power  usually  manifests  itself  during  maturity.  Among  animals 
an  offspring  may  originate  from  its  parent  by  asexual  or  sexual 
methods.  Reproduction  may  take  place  by  a  splitting  of  the 
entire  body  into  two  or  more  parts,  each  of  which  takes  up  an 
independent  existence.  This  method  of  multiplication  is  often 
called  asexual  reproduction;  if  the  new  individuals  are  formed  by  a 
division  of  the  original  or  parent  body  into  approximately  equal 
parts,  reproduction  is  said  to  be  by  fission;  if,  on  the  other  hand, 
only  a  small  portion  of  the  parent  individual  becomes  separated 
as  a  distinct  organism,  reproduction  is  by  budding,  and  the  off- 
spring are  called  bads.  Sometimes  an  animal  passes  through  a 
resting  stage  during  which  the  body  fragments  into  a  large  num- 
ber of  small  parts  which  subsequently  become  free  individuals. 
This  asexual  method  of  reproduction  is  known  as  sporulation. 

Sexual  reproduction  involves  the  union  of  substances  from  two 
different  animals.  When  this  process  occurs  in  certain  one-celled 
animals,  two  individuals  come  together  and  exchange  a  portion 
of  their  living  substance  (Fig.  31);  in  many-celled  animals  there 
is  a  union  of  sexual  cells,  egg  and  sperm,  which  originate  from 
two  separate  individuals.  Asexual  reproduction  increases  the 
number  of  individuals  without  altering  the  nature  of  their 
substance;  sexual  reproduction  originates  new  individuals  by 
mixing  portions  of  two  preexisting  organisms. 

Asexual  reproduction  may  be  illustrated  by  Paramecium  (p.  67), 
a  single-celled  animal  which  divides  twice  during  every  twenty- 
four  hours  when  conditions  are  favorable.  Its  metabolism  is  so 
rapid  that  it  can  grow  to  its  maximum  size  in  half  that  period  of 
time.  The  honey  bee  (Chap.  XII)  furnishes  an  excellent  illus- 
tration of  sexual  reproduction.  Three  sorts  of  bees  exist:  the 


16  AN  INTRODUCTION  TO  ZOOLOGY 

workers,  which  are  sterile  females  and  take  no  part  in  repro- 
duction; queens,  or  sexually  mature  females;  and  drones,  or 
males.  New  individuals  are  formed  by  the  fusion  of  reproduc- 
tive or  germ  cells.  The  drone  injects  the  sperm  cells  into  the 
queen,  where  they  are  stored  up  to  unite  with  the  egg  cells  as 
they  ripen  within  her  body. 

(7)  Irritability  or  reactiveness  is  a  characteristic  property  of 
organisms.  It  gives  them  the  power  to  respond  to  changes  in 
their  environment  in  such  a  way  that  they  usually  attain  favorable 
conditions  for  existence.  Any  change  in  conditions  that  pro- 
duces the  reactions  of  an  animal  is  termed  a  stimulus.  Stimuli 
may  be  external  or  internal;  the  former  includes  changes  in 
temperature  or  light,  in  the  composition  of  the  surrounding  me- 
dium, etc. ;  hunger  and  fatigue  may  serve  as  examples  of  internal 
stimuli. 

In  many  instances  animals  show  very  uniform  responses  to 
stimuli.  An  excellent  illustration  of  this  fact  is  furnished  by  the 
katydid.  The  relation  between  the  temperature  and  the  number 
of  calls  per  minute  is  so  constant  in  this  insect  that  the  exact 
atmospheric  temperature  may  be  computed  from  the  following 
formula. 

,  no.  of  calls  per  minute  —  IQ 
Temperature  =  60  +  - 

o 

The  adaptive  power  of  animals  may  be  further  illustrated  by 
the  kidneys  of  man.  In  man,  as  in  all  vertebrates,  these  are 
paired  organs  occupying  a  position  near  the  dorsal  side  of  the 
body  wall.  Under  normal  conditions  both  eliminate  an  approxi- 
mately equal  amount  of  excretory  matter.  However,  no  perma- 
nent injury  necessarily  results  if  one  is  removed,  for  the  kidney  on 
the  uninjured  side  of  the  body  enlarges  to  such  an  extent  that 
it  is  able  to  carry  on  the  functions  previously  performed  by  both. 

Instances  of  this  adaptive  power  might  easily  be  multiplied,  for 
they  are  common  everywhere.  In  a  broad  sense,  even  the  in- 
telligence of  man  and  the  degree  of  success  he  may  attain  depend 
upon  how  successfully  his  reactions  meet  surrounding  conditions. 


PHENOMENA  OF  LIFE  17 

3.  PLANTS  AND  ANIMALS  COMPARED 

It  is  easy  to  choose  characteristics  that  will  serve  to  distinguish 
a  tree  from  a  man,  but  the  separation  of  the  simplest  animals 
from  the  simplest  plants  is  a  more  difficult  problem.  In  fact, 
there  are  at  the  present  time  a  number  of  organisms  that  are 
claimed  by  both  botanists  and  zoologists.  There  is  no  single 
peculiarity  which  can  be  used  in  all  cases  to  discriminate  between 
these  groups  of  organisms.  The  view  now  generally  accepted 
is  that  plants  and  animals  originated  together  but  have  developed 
along  divergent  lines.  However,  certain  general  features  can 
be  indicated  in  which  the  two  kingdoms  differ.  These  are  given 
in  the  following  table;  but  the  reader  should  bear  in  mind  that 
there  are  exceptions  to  every  one  of  these  criteria. 


TABLE  II 

CHIEF  CHARACTERISTICS  OF  PLANTS  AND  ANIMALS  CONTRASTED 
PLANTS  ANIMALS 

1.  Structure  Form  of  body  rather  vari-  Form  of  body  usually  inva- 

able  ;    new  organs  added      riable ;    organs  compact 
externally.  and  mostly  internal. 

2.  Locomotion          Usually  none  in  adult  con-  Usually  well  developed. 

dition. 

3.  Irritability  Respond  to  stimuli  slowly ;  Respond  to   stimuli  quick- 

no  nervous  system.  ly ;  nervous  system  pres- 

ent in  higher  forms. 

4    Metabolism          Possess  chlorophyll ;    manu-   No    chlorophyll ;     require 
facture  organic  food  from      organic  food. 
CO-2  and  H2O  in  the  pres- 
ence of  light. 

5.  Waste  products    Oxygen,    carbon    dioxide,    Carbon  dioxide,  water,  urea, 
water.  faeces. 

The  presence  of  chlorophyll  in  green  plants  has  an  important 
bearing  on  metabolism,  for  it  enables  plants  to  manufacture 
starch  when  light  is  available.  This  simple  organic  compound  is 


18  AN  INTRODUCTION  TO  ZOOLOGY 

built  from  the  inorganic  substances,  carbon  dioxide  and  water. 
With  starch  as  a  basis  more  complex  organic  compounds  are 
elaborated  by  the  living  substances.  Now  the  living  substance 
of  plants  is  practically  the  same  as  that  of  animals,  and  it  seems 
strange  at  first  thought  that  plants  should  give  off  oxygen  as  a 
waste  product,  for  this  gas  is  one  of  the  constant  needs  of  living 
substance.  If,  however,  we  keep  in  mind  the  fact  that  chloro- 
phyll is  carrying  on  one  set  of  processes  (Fig.  3)  which  require 


CARBON  DIOXIDE 


FIG.  3.   Diagram  showing  the  activities  of  chlorophyll. 

carbon  dioxide  and  liberate  oxygen  as  a  waste  product,  while 
the  living  substance  is  carrying  on  another  set  which  use  oxygen 
and  release  carbon  dioxide  (Fig.  2),  the  matter  becomes  one  of 
simple  addition  and  subtraction;  one  group  of  processes  uses  the 
waste  products  of  the  other. 

The  qualities  that  are  usually  cited  as  being  peculiarly  charac- 
teristic of  animals  are  locomotion  and  nervous  activity.  With  the 
exception  of  a  few  extremely  sensitive  species  of  which  the  com- 
mon sensitive  plant,  Mimosa  pudica,  is  the  most  familiar  ex- 
ample, plants  respond  very  slowly  to  external  stimuli  and  their 
power  of  transmitting  impulses  is  poorly  developed.  Locomotion 
is  absent  except  in  a  few  simple  forms  and  free  swimming  repro- 
ductive cells. 


PHENOMENA  OF  LIFE  1 9 

4.  THE  PHYSICAL  BASIS  OF  LIFE:  PROTOPLASM 

When  the  biologists  of  the  past  century  turned  their  micro- 
scopes upon  the  swarms  of  simpler  plants  and  animals  or  exam- 
ined the  tissues  of  multicellular  organisms,  they  saw  the  remark- 
able substance  that  forms  the  physical  basis  for  all  the  activities 
of  living  organisms  without  realizing  its  importance.  Felix 
Dujardin,  in  1835,  first  clearly  distinguished  this  substance 
from  other  viscid  matter  and  called  it  "  sarcode."  Later  inves- 
tigators made  more  extended  studies,  but  Dujardin's  term  was 
not  generally  adopted.  Sarcode  was  believed  to  be  generally 
present  in  the  simple  animals,  but  a  different  substance,  called 
protoplasm,  was  thought  to  be  the  living  element  in  the  complex 
organisms.  Max  Schultze,  in  1861,  convinced  the  scientific 
world  that  the  living  substance  in  both  simple  and  complex 
plants  and  animals  is  practically  identical  in  structure,  compo- 
sition, and  physiological  properties.  The  term  "  protoplasm  " 
adopted  at  that  time  has  persisted  to  the  present  day. 

Schultz's  generalizations  were  of  vast  importance  and  gave  a 
great  impetus  to  the  study  of  fundamental  problems.  Huxley 
called  protoplasm  the  physical  basis  of  life,  a  peculiarly  expres- 
sive phrase,  since  in  protoplasm  lies  the  ultimate  explanation 
of  all  vital  phenomena.  It  is  fitting,  therefore,  in  this  connection 
to  consider  more  fully  the  properties  of  living  substance. 

Protoplasm  is  subject  to  all  the  physical  laws  of  fluids.  It 
exhibits  currents  and  flowing  movements,  and  is  influenced  by 
changes  in  surface  tension.  Its  specific  gravity  is  always  a  trifle 
greater  than  that  of  water.  It  is  usually  entirely  colorless  or 
gray.  On  account  of  its  importance,  the  minute  structure  of 
this  living  jelly  has  long  been  a  favorite  field  for  study  in  both 
plants  and  animals.  Protoplasm  has  been  repeatedly  examined 
under  the  very  highest  powers  of  the  microscope.  Elaborate 
methods  have  been  devised  for  preserving  it  so  that  its 
structure  may  be  similar  to  that  which  existed  during  life. 
Many  theories  have  been  advocated  with  regard  to  the  finer 


20  AN  INTRODUCTION  TO  ZOOLOGY 

structure  of  protoplasm.     Of  these  the  three  following  may  be 
mentioned. 

(1)  The  Reticular  Theory  maintains  that  protoplasm  consists 
of  a  living  network  of  anastomosing  fibers  (Fig.  4,  B) ;  between 
these  a  variety  of  non-living  substances,  such  as  water  and  fat, 
may  be  present. 

(2)  The  Alveolar  Theory  (Biitschli,  1892)  supposes  that  pro- 
toplasm has  a  foam-like  structure  somewhat  similar  to  a  mass  of 
minute  droplets  such  as  exist  in  an  emulsion  of  oil  and  water 

(Fig.  4,  A).  The  originator 
of  this  theory  was  able  to 
make  artificial  emulsions 
which  showed  a  striking 
resemblance  to  living  sub- 
stance. He  maintained  that 
the  fibrous  network,  de- 
scribed bv  the  adherents  of 

A  "R 

FIG.  4.  Diagrams  illustrating  (A)  the  alve-  the  reticular  theory,  repre- 
olar,  and  (B)  the  reticular  theory  of  sented  the  walls  of  the  alvC- 
protoplasmic  structure.  (From  Dahl-  olar  bodies  which  had  been 
gren  and  Kepner.)  cut  acrOss. 

(3)  The  Granular  Theory  (Altman,  1892)  asserts  that  proto- 
plasm is  composed  of  innumerable  living  granules  arranged  either 
along  fibers  or  among  alveoli;  nothing  is  essential,  however,  ex- 
cept the  granules. 

None  of  these  theories  has  universal  approval,  but  the  first  two 
have  been  more  widely  accepted  than  the  last.  Many  recent 
authorities  believe  that  protoplasm  is  highly  variable  in  structure. 
A  fourth  view  held  by  many  observers  is  "  that  the  various  types 
described  above  are  connected  by  intermediate  gradations  and 
may  be  transformed  one  into  another  "  (19,  p.  27). 

Protoplasm  is  a  mixture  of  extremely  complex  chemical  com- 
pounds; these  are,  at  least  in  part,  extremely  labile  and  therefore 
are  apt  to  vary  at  different  times.  It  is  impossible  to  study 
living  substance,  since  even  the  most  careful  methods  of  analysis 


PHENOMENA  OF  LIFE  21 

immediately  kill  protoplasm.  "  Hence  ideas  upon  the  chemistry 
of  the  living  object  can  be  obtained  only  by  deductions  from 
chemical  discoveries  in  the  dead  object"  (17,  p.  102).  Never- 
theless, we  have  gained  a  considerable  amount  of  knowledge  as  to 
the  chemical  nature  of  protoplasm.  Ninety-seven  per  cent  of 
the  organic  and  inorganic  constituents  of  protoplasm  are  made 
up  of  four  elements,  occurring  in  the  following  proportions:  — 

Oxygen 65  per  cent 

Carbon 18.5  per  cent 

Hydrogen       n  per  cent 

Nitrogen 2.5  per  cent 

These  and  other  less  important  elements  form  certain  rather 
definite  groups  of  compounds,  which  we  will  now  consider  in  some 
detail. 

Water  is  the  most  important  of  the  inorganic  constituents; 
it  comprises  more  than  50  per  cent  of  the  weight  of  most 
animals,  and  in  some  marine  forms  reaches  as  high  as  99  per 
cent.  It  seems  remarkable  that  man  is  able  to  exist  when  59 
per  cent  of  his  body  is  made  up  of  water.  It  not  only  occurs  in 
combinations,  but  also  acts  as  a  solvent  for  various  substances 
that  are  found  in  protoplasm. 

Aside  from  water  the  most  important  inorganic  substances  are 
various  salts  and  dissolved  gases.  Of  the  salts  the  chlorides, 
carbonates,  and  phosphates  of  the  commoner  alkali  and  alkali- 
earth  metals  predominate.  The  principal  gases  are  carbon  di- 
oxide and  oxygen. 

The  organic  compounds  found  in  protoplasm  are  divided  into 
three  general  classes:  proteids,  carbohydrates,  and  fats.  "'Of 
these  only  the  proteids  and  their  derivatives  have  been  demon- 
strated with  certainty  as  common  to  all  cells;  hence,  they  must 
be  set  apart  among  the  organic  constituents  of  living  matter  as 
the  essential  or  general  substances,  in  contrast  to  all  special 
substances  "  (17,  p.  103). 

Proteids  are  always  composed  of  the  five  elements,  carbon, 


22  AN  INTRODUCTION  TO  ZOOLOGY 

oxygen,  hydrogen  and  nitrogen,  and  a  small  amount  of  sulphur. 
In  this  respect  they  differ  from  the  other  two  classes  of  organic 
compounds  which  lack  the  nitrogen  and  sulphur.  The  proteid 
molecules  are  extremely  large,  one  often  containing  more  than 
a  thousand  atoms.  They  are  not  readily  diffusible  through 
animal  membranes,  this  being  due  in  part  to  the  size  of  the  mole- 
cules and  also  to  the  fact  that  such  complex  compounds  do  not 
dissolve  completely  unless  they  break  down  into  simple  sub- 
stances. Instead  of  dissolving,  proteids  absorb  enormous 
quantities  of  water,  swelling  up  like  a  sponge ;  they  are  called 
colloids  to  distinguish  them  from  crystalloids,  like  sugar,  which 
are  easily  soluble.  Coagulation,  or  clotting,  is  another  peculiarity 
of  proteids. 

Carbohydrates  are  compounds  of  carbon,  hydrogen,  and  oxygen, 
the  last  two  always  occurring  in  the  same  proportions  in  which 
they  are  found  in  water  (H2O).  The  most  familiar  examples 
of  this  class  of  substances  are  the  starches  and  sugars,  which  are 
characteristic  of  plants  rather  than  animals,  though  found  in  both. 
Carbohydrates  are  comparatively  simple  compounds  when 
contrasted  with  the  proteids,  and  are  readily  oxidizable,  thus 
producing  energy.  Some  varieties  of  living  substance  apparently 
contain  no  carbohydrates. 

Fats,  likewise,  are  not  invariable  constituents  of  protoplasm. 
Though  of  widespread  occurrence,  fatty  compounds  are  particu- 
larly characteristic  of  animals.  They  may  be  said  to  consist  of 
an  alcohol  (glycerin)  which  has  lost  some  water  and  combined 
with  a  fatty  acid.  They  are  all  lighter  than  water,  and  do  not 
unite  with  it. 

Protoplasm  consists,  then,  of  complex  and  variable  mixtures  of 
proteids,  carbohydrates,  and  fats,  together  with  water,  salts,  and 
dissolved  gases.  No  uniform  chemical  formula  can  be  given  for 
it;  in  fact,  one  of  its  chief  characteristics  lies  in  the  unceasing 
changes  that  it  undergoes.  It  has  the  properties  of  metabolic 
activity,  irritability,  contractility,  reproduction,  and  all  others 
peculiar  to  living  organisms  (p.  10).  It  seems  strange  that  such 


PHENOMENA  OF  LIFE  23 

a  changeable  substance  should  possess  great  specificity,  yet  such 
is  the  case.  The  protoplasm  of  each  species  of  organism  is  per- 
fectly distinct  from  that  of  every  other  species.  A  tree  of  a 
certain  variety  always  gives  rise  to  other  trees  similar  to  it ; 
for  example,  a  pear  seed  never  develops  into  a  peach  tree, 
but  always  into  a  pear  tree  of  the  same  kind  that  produced  it; 
likewise  the  egg  of  a  pure-bred  Plymouth  Rock  hen  never  gives 
rise  to  anything  but  a  chicken  of  the  Plymouth  Rock  variety. 
Nevertheless,  both  the  tree  and  the  hen  take  their  origin  from 
an  apparently  simple  microscopic  bit  of  protoplasm,  the  fertilized 
germ  cell. 

5.  A  PHYSICO-CHEMICAL  EXPLANATION  OF  THE  PHENOMENA 
OF  LIFE 

The  efforts  of  the  alchemist  of  old  who  wrought  in  the  seclusion 
of  his  laboratory  were  directed  toward  the  solution  of  two  prob- 
lems: the  production  of  an  elixir  of  life,  and  the  manufacture  of 
gold.  His  time  has  passed,  and  his  problems  were  never  solved. 
His  successor  of  modern  times,  the  biochemist,  has  likewise 
chosen  problems  of  fundamental  importance  and  great  difficulty; 
he  has  attempted  to  give  an  explanation  of  all  vital  phenomena 
by  means  of  physical  and  chemical  laws.  The  scientific  world 
now  contains  many  scholars  who  maintain  that  living  organisms 
are  really  machines,  and  are  opposed  to  the  idea  of  vitalism,  which 
presupposes  the  presence  of  some  vital  principle.  One  of  the 
leading  investigators  in  this  field  claims  that  living  organisms  are 
to  be  considered  as  "chemical  machines,  consisting  essentially 
of  colloidal  material,  which  possess  the  peculiarities  of  auto- 
matically developing,  preserving,  and  reproducing  themselves  " 
(15,  p.  i).  Another  prominent  zoologist  makes  the  following 
statement:  "  I  say  again,  with  all  possible  emphasis,  that  the 
mechanistic  hypothesis  or  machine  theory  of  living  beings  is  not 
fully  established,  that  it  may  not  be  adequate  or  even  true;  yet  I 
can  only  believe  that  until  every  other  possibility  has  really  been 


24  AN  INTRODUCTION  TO  ZOOLOGY 

exhausted,  scientific  biologists  should  hold  fast  to  the  working 
program  that  has  created  the  sciences  of  biology.  The  vitalistic 
hypothesis  may  be  held,  and  is  held,  as  a  matter  of  faith;  but  we 
cannot  call  it  science  without  misuse  of  the  word"  (20,  p.  21). 
A  great  many  scientists  have  recently  come  forward  to  combat 
the  old  idea  of  vitalism,  and,  though  some  of  them  have  made 
errors,  their  influence  as  a  whole  has  been  highly  stimulating  to 
zoological  thought. 

Until  comparatively  recent  times  it  was  believed  that  organic 
compounds  could  be  built  up  only  by  living  substance.  Even 
such  an  able  scientist  as  Liebig  supposed  that  his  beef  extract 
contained  some  vitalistic  principle  that  was  imparted  to  those 
who  ate  it.  His  ideas  were  overthrown  by  a  simple  experiment. 
Six  puppies  from  the  same  litter  were  divided  into  two  lots; 
three  were  fed  beef  extract  and  the  others  were  allowed  to  remain 
without  any  food  whatever.  Those  fed  with  the  beef  extract 
died  first,  thus  demonstrating  that  the  extract  was  more  a  stimu- 
lant than  a  food.  The  belief  that  organic  compounds  could 
only  come  from  living  organisms  was  proven  erroneous  as  early 
as  1828,  when  Woehler  made  urea  by  a  synthetic  process  in  his 
laboratory.  It  was  to  combat  such  mtalistic  ideas  that  the 
"  mechanistic  "  theory  first  arose,  and  its  adherents  have  met 
with  a  considerable  degree  of  success.  Experiments  from  many 
sources  might  be  cited  which  support  the  mechanistic  view,  but 
only  a  few  selected  instances  can  be  given. 

Many  organs  will  carry  on  their  normal  activities  when  removed 
from  an  animal's  body.  A  turtle's  heart  will  remain  alive  and 
beat  for  a  couple  of  days  if  it  is  placed  in  a  dish  containing  a 
proper  mixture  of  mineral  salts  in  water.  Furthermore,  the  rate 
of  the  beats  may  be  increased  or  decreased  at  the  will  of  the  experi- 
menter by  varying  the  proportionate  amounts  of  the  salts  in 
solution. 

Organs  from  one  animal  may  be  grafted  upon  another.  There 
is  a  gland,  the  thyroid,  in  the  neck  of  human  beings,  which,  when 
diseased,  causes  a  malady  known  as  goiter.  The  removal  of  the 


PHENOMENA  OF  LIFE  25 

thyroid  is  followed  by  death.  It  was  discovered  some  time  ago 
that  this  gland  might  be  removed  with  impunity  if  a  piece  of  a 
healthy  gland  from  another  animal,  such  as  a  sheep,  were  grafted 
under  the  skin  in  any  part  of  the  body.  The  foreign  piece  of 
tissue  appropriates  a  blood  supply  and  becomes  functional  in  its 
new  position.  This  is  one  of  the  many  cases  proving  that  a 
definite  positional  relation  of  organs  is  not  always  necessary  if 
their  secretions  are  present. 

The  opponents  of  vitalism  have  also  brought  forward  many 
arguments  against  that  theory  from  a  study  of  enzymes  or  fer- 
ments, a  class  of  substances  which  cause  chemical  reactions  with- 
out undergoing  any  change  themselves.  An  example  is  ptyalin, 
which  is  present  in  human  saliva  and  has  for  its  function  the 
change  of  starch  to  sugar.  A  large  number  of  substances  of  this 
sort  are  known  to  be  present  in  living  organisms.  The  explana- 
tion of  their  actions  makes  clear  many  processes  which  were 
formerly  supposed  to  be  due  to  certain  vitalistic  principles.  Re- 
search in  this  field  is  only  in  its  infancy. 

These  instances  are  sufficient  to  show  that  the  mechanistic 
point  of  view  is  a  much  more  progressive  one  than  the  vitalis- 
tic. Through  its  influence  scientists  have  been  able  to  prove 
that  a  large  number  of  the  activities  present  in  living  matter  are 
subject  to  definite  laws,  many  of  which  have  been  known  for  a 
long  time  to  physicists  and  chemists.  However,  this  method  has 
not  explained  all  vital  phenomena,  and  perhaps  never  will. 


CHAPTER  III 
THE   CELL  AND   THE   CELL  THEORY 

i.  MORPHOLOGY  AND  PHYSIOLOGY  OF  CELLS 

THE  protoplasm  contained  in  the  body  of  an  animal  is  not  one 
continuous  mass,  but  is  separated  by  membranes  and  other 
structures  into  a  great  number  of  minute  bodies  called  cells. 
These  cells,  though  varying  greatly  in  size  and  shape,  all  have 
the  same  fundamental  plan  of  structure.  The  simplest  animals 
consist  of  a  single  cell,  but  the  bodies  of  the  more  complex  forms 
are  made  up  of  millions  of  these  tiny  bits  of  protoplasm.  On 
account  of  its  universal  presence  in  animals  and  plants,  the  cell 
has  been  called  the  unit  of  life.  It  is  necessarily  of  vast  im- 
portance in  the  solution  of  biological  problems,  and  has  been 
much  studied  by  investigators  during  the  past  forty  years. 

In  order  to  have  the  structure  of  a  typical  cell  clearly  in  mind 
we  will  now  turn  our  attention  to  the  following  description  of  its 
parts.  Reference  to  Figure  5  will  make  the  written  description 
clearer.  Since  the  cell  contains  protoplasm,  which  acts  like  a 
fluid,  it  naturally  tends  to  have  a  spherical  form.  This  condition 
is  seldom  realized  in  nature,  for  various  factors,  such  as  unequal 
growth  and  pressure  from  other  cells,  modify  the  shape.  The 
fact  that  the  cell  shown  in  the  figure  has  an  oblong  outline  is, 
therefore,  of  no  significance. 

The  contents  of  many  cells  includes  not  only  the  active  proto- 
plasm, but  also  various  kinds  of  passive  bodies  such  as  fat  glob- 
ules. These  are  known  as  metaplasm.  The  protoplasm  of  any 
cell  is  made  up  of  two  principal  kinds  of  substance,  the  cytoplasm 
and  the  karyoplasm.  The  nucleus  contains  all  of  the  latter.  It 
is  the  central  body,  and  is,  for  various  reasons,  regarded  as  the 

26 


THE  CELL  AND  THE  CELL  THEORY 


controlling  organ  of  cell  activities.  A  distinct  though  delicate 
membrane  usually  separates  it  from  the  surrounding  cytoplasm. 
The  ground  substance  of  the  nucleus  is  a  sort  of  sap,  called 
nucleoplasm,  through  which  runs  a  network  of  thin  limn  fibers. 
The  most  important  nuclear  constituent  is  the  chromatin,  a  sub- 
stance that  has  a  strong  affinity  for  certain  dyes.  Chromatin 

Attraction-sphere  enclosing 
two  centrosomes 


Plasmosome  or 
true  nucleolus 

rhromatin  net- 
work 

Linin  network 


Karyosome, 
net-knot,  or 
chromatin- 
nucleolus 


FIG.  5.     Diagram  of  a  cell.     (From  Wilson.) 

is  generally  arranged  in  the  form  of  a  more  or  less  irregular  net- 
work of  granules  which  are  scattered  about  on  the  linin  fibers. 
Frequently  several  granules  unite  to  form  a  net  knot  or  karyo- 
some.  In  addition  to  these  regular  constituents  of  the  nucleus, 
one  or  more  bodies,  known  as  nudeoli,  may  be  present.  Al- 
though cells  usually  contain  only  one  nucleus,  two  or  more  may 
occur  in  certain  cases. 
Cytoplasm  always  surrounds  the  nucleus.  It  is  sometimes 


28  AN  INTRODUCTION  TO  ZOOLOGY 

arranged  in  concentric  zones,  each  containing  a  different  kind  of 
protoplasm;  or  it  may  be  modified  to  form  hair-like  projections 
called  cilia,  and  other  cellular  organs.  The  structure  of  cyto- 
plasm is,  however,  rather  uniform  in  the  same  cell,  but  highly 
variable  in  different  cells,  and  sometimes  even  in  different  stages 
of  an  individual  cell.  A  careful  examination  of  certain  cells 
appears  to  leave  no  doubt  but  that  a  reticular  network  is  present 
in  the  cytoplasm.  Other  cells  show  no  sign  of  such  an  arrange- 
ment of  materials.  The  alveolar  condition  exhibited  by  many 
cells  is  perhaps  the  most  typical. 

Although  a  definite  cell  wall  is  more  often  found  in  plants  than 
in  animals,  the  cytoplasm  of  the  latter  may  secrete  a  delicate 
membrane,  or  even  a  wall  of  considerable  thickness,  as  in  the  case 
of  cartilage  (Fig.  48,  C). 

Embedded  in  the  cytoplasm,  usually  near  the  nucleus,  is  a 
minute  but  important  body,  the  centrosome,  or  a  pair  close  to- 
gether. The  centrosomes  are  often  surrounded  by  a  clear  space, 
the  centrosphere.  Among  the  other  bodies  suspended  in  the  cyto- 
plasm may  be  mentioned  oil  and  water  vacuoles,  crystals,  and  cer- 
tain organs  known  as  plastids.  Plastids  may  be  colored  (chromo- 
.plastids)  or  white  (leucoplastids). 

There  is  a  definite  division  of  labor  among  the  parts  of  a  cell. 
The  particular  function  of  the  nucleus,  aside  from  its  important 
relation  to  cell  division,  to  be  described  later,  seems  to  be  the 
control  of  the  activities  by  which  the  protoplasm  is  elaborated. 

The  cytoplasm,  from  its  direct  relation  to  the  outside  world,  is 
the  seat  of  such  functions  as  irritability,  absorption,  digestion, 
excretion,  and  respiration.  The  centra  some  is  of  importance 
during  cell  division.  The  cell  covering  may  be  secreted  for  pro- 
tection or  support,  or  may  be  extremely  delicate  and  have  sig- 
nificance only  as  it  helps  to  control  the  absorption  of  certain 
fluids.  Plastids  may  represent  stored  food  or  waste  products; 
some  of  them,  however,  have  other  functions,  e.g.,  the  chloroplasts, 
which  carry  on  photosynthesis  in  many  plant  and  a  few  animal 
cells. 


THE   CELL  AND  THE   CELL  THEORY  29 

As  has  already  been  noted,  the  size  arid  form  of  cells  are  variable. 
Bacteria  are  among  the  smallest  known,  some  of  them  measuring 
not  more  than  one  hundred  thousandth  of  a  meter  in  length. 
On  the  other  hand,  the  eggs  of  a  large  bird  are  single  cells;  they 
are  exceeded  in  length  by  certain  nerve  cells  whose  fibers  extend 
.out  almost  a  meter.  The  limit  in  size  is  probably  determined 
by  the  nucleus,  since  it  regulates  the  metabolic  activities  of  the 
cell. 

Some  idea  of  the  wide  variation  in  the  form  of  cells  may  also 
be  gained  from  Figure  48,  which  shows  specimens  of  various  types. 

All  of  the  life  processes  of  the  simplest  animals,  the  Protozoa, 
are  carried  on  by  a  single  cell.  The  cells  of  a  multicellular  ani- 
mal, a  Metazoon,  are  mutually  dependent,  and  cannot  exist  if 
isolated.  In  many  cases  there  is  actual  continuity  between  the 
protoplasm  of  different  cells,  and  some  zoologists  have  gone  so 
far  as  to  maintain  that  the  body  of  a  complex  animal  should  not 
be  considered  an  assemblage  of  separate  cells,  but  a  continuous 
mass  of  protoplasm  with  nuclei  scattered  through  it.  Often 
this  connection  between  cells  is  brought  about  by  slender  proto- 
plasmic bridges  (Fig.  46,  C) ;  in  other  instances  cells  are  so  indis- 
tinguishably  blended  that  no  boundaries  can  be  discerned.  A 
cell  is  therefore  not  always  a  true  "  unit,"  but  a  necessary  part 
in  a  complex  whole.  The  number  of  cells  associated  to  form 
a  single  organ  is  often  enormous;  the  number  of  nerve  cells  in 
the  gray  matter  of  the  human  brain  has  been  estimated  at 
9,200,000,000;  all  of  these  together  would  occupy  only  a  trifle 
more  than  a  cubic  inch  of  space. 

2.  CELL  DIVISION 

Cells  multiply  either  by  direct  or  indirect  division.  Indirect 
cell  division  always  involves  a  definite  series  of  stages  which 
follow  each  other  in  a  regular  sequence,  the  whole  process  being 
known  as  mitosis,  or  mitotic  cell  division.  Direct  division  or 
amitosis,  is  a  more  simple  process  having  no  complex  series  of 
stages. 


AN  INTRODUCTION  TO  ZOOLOGY 


(i)  Mitosis.  —  A  cell  that  is  not  undergoing  division  is  said 
to  be  in  a  resting  condition  (Fig.  6,  A).     When  it  divides  by  the 

indirect  method,  the  se- 
ries of  changes  may  con- 
veniently be  arranged  in 
four  stages,  known  as  the- 
prophase,  metaphase,  ana- 
phase,  and  telophase.  The 
modifications  occurring 
during  mitosis  are  prima- 
rily concerned  with  the 
nucleus  and  centrosomes, 
the  cytoplasm  remaining 
comparatively  passive. 

The  prophase  is  char- 
acterized by  preparatory 
changes.  The  chromatin 
granules  that  are  scat- 
tered through  the  nucleus 
in  the  resting  cell  become 
arranged  in  the  form  of 
a  long  thread  or  spireme 
(Fig.  6,B).  At  the  same 
time  the  centrosomes 
move  apart,  finally  reach- 
ing opposite  sides  of  the 
nucleus  (B,  C,  D,  E). 
The  radiating  lines  that 
appear  about  them  (B) 
later  give  rise  to  a  spindle 
(D).  While  this  is  going 
FIG.  6.  Diagrams  illustrating  mitotic  cell  on  the  nuclear  membrane 
division.  (From  Wilson.)  generally  disintegrates 

and  the  spireme  segments  into  a  number  of  bodies  called  chro- 
mosomes (D) ;    these  take  a  position  at  the  equator  of  the  spindle. 


THE  CELL  AND  THE   CELL  THEORY  31 

halfway  between  the  centrosomes  (E).  The  stage  shown  in 
Figure  6,  F,  is  known  as  the  amphiaster ;  at  this  time  all  of 
the  machinery  concerned  in  mitosis  is  present.  There  are  two 
asters,  each  consisting  of  a  centrosome  surrounded  by  a  number 
of  radiating  astral  rays,  and  a  spindle  which  lies  between  them. 
The  chromosomes  lie  in  the  equatorial  plate  (ep). 

During  the  second  stage,  the  metaphase,  the  chromosomes  split 
in  such  a  way  that  each  of  their  parts  contains  an  equal  amount 
of  chromatin  (G).  As  we  shall  see  later,  this  is  one  of  the  most 
significant  events  that  takes  place  during  mitosis.  Often  the 
chromosomes  split  before  they  have  assumed  an  equatorial  posi- 
tion (Fig.  6,  E) ;  other  minor  variations  also  occur. 

During  the  anaphase  (Fig.  6,  H),  the  chromosomes  formed 
by  splitting  move  along  the  spindle  fibers  to  the  centrosomes. 
As  a  result  every  chromosome  present  at  the  end  of  the  prophase 
(F)  sends  half  of  its  chromatin  to  either  end  of  the  spindle.  The 
mechanism  that  brings  about  this  migration  is  as  yet  somewhat 
in  question.  Fibers  are  usually  left  between  the  separating 
chromosomes;  these  are  known  as  inter  zonal- fibers  (H,if). 

The  telophase  (Fig.  6,  I,  J)  is  a  stage  of  reconstruction  from 
which  the  nuclei  emerge  in  a  resting  condition;  the  chromatin 
becomes  scattered  throughout  the  nucleus,  which  is  again  envel- 
oped by  a  definite  membrane  (J) ;  the  centrosome  divides  and, 
with  the  centrosphere,  takes  a  position  near  the  nucleus.  Finally 
the  cycle  is  completed  by  the  constriction  of  the  cell  into  two 
daughter  cells  (I,  J)  each  of  which  is  in  condition  to  carry  on  the 
regular  metabolic  processes  until  it  in  turn  becomes  ready  to 
divide. 

The  origin  of  the  structures  that  take  an  active  part  in  mitosis 
is  not  definitely  known.  Only  the  centrosomes  are  represented 
in  the  resting  cell;  these  usually  arise  from  the  division  of  pre- 
existing centrosomes,  but  in  certain  cases  they  are  wholly  absent 
(higher  plants),  or  their  existence  is  questionable  (Protozoa). 
The  chromosomes  are  formed  directly  by  a  condensation  of  chro- 
matin. The  origin  of  the  spindle  fibers  is  variable.  Sometimes 


32  AN  INTRODUCTION  TO  ZOOLOGY 

they  are  formed  from  the  cytoplasm,  and  sometimes  they  arise 
from  the  linin  network  in  the  nucleus.  Linin  does  not,  however, 
differ  greatly  in  chemical  composition  from  the  cytoplasmic 
substance,  and  the  spindle  fibers  are,  therefore,  always  composed 
of  achromatic  material.  The  nudeolus  is  apparently  of  no  impor- 
tance in  mitosis;  it  degenerates  during  the  early  stages  (Fig.  6, 
G,  H,  n)  and  is  reformed  during  the  telophase  (J).  The  two  cells 
that  result  from  mitosis  may  be  equal  (J)  or  unequal  in  size.  The 
division  of  the  cytoplasm  appears  to  be  of  little  importance,  but 
with  rare  exceptions  there  seems  to  be  an  equal  division  of 
chromatin;  this  is  apparently  the  essential  process. 

Under  ordinary  conditions  every  animal  develops  from  a  single 
cell,  and,  since  the  chromatin  persists  from  one  cell  generation  to 
another,  the  chromosomes  are  considered  by  most  zoologists  to 
be  the  bearers  of  hereditary  qualities  from  parent  to  offspring. 
Every  species  of  animal  has  a  definite  number  of  chromosomes 
that  appear  when  the  cells  of  its  body  undergo  mitosis.  Thus 
sixteen  are  characteristic  of  the  cells  of  oxen,  guinea  pigs,  and  man; 
the  grasshopper  has  twelve;  the  brine  shrimp  (Artemia),  one 
hundred  and  sixty  eight;  the  round  worm  (Ascaris),  four  or  two. 
The  last  example  illustrates  one  of  the  unusual  cases  in  which  two 
individuals  that  are  mutually  fertile  have  a  different  number  of 
chromosomes.  An  even  number  of  chromosomes  is  characteristic 
of  most  animals,  but  recent  researches  have  demonstrated  that 
some  forms,  particularly  the  males  of  insects,  have  an  odd 
number. 

(2)  Amitosis.  —  Although  mitosis  is  considered  the  typical 
method  of  cell  division,  the  multiplication  of  nuclei  by  direct 
division  or  amitosis  is  not  uncommon.  This  process  (Fig.  7) 
does  not  have  such  a  definite  series  of  stages  as  mitosis,  and  is 
much  simpler.  The  nucleus  elongates,  and  the  nucleolus  di- 
vides, one  half  passing  to  either  end  of  the  nucleus.  Sometimes 
the  nucleolus  remains  at  one  end,  a  new  one  being  formed  at  the 
opposite  end.  The  nucleus  divides  by  one  of  three  methods: 
(a)  often  the  nucleus  is  pinched  in  two  in  the  middle,  or  (b)  a 


THE  CELL  AND  THE  CELL  THEORY 


33 


plate  is  formed  at  the  plane  of  division  which  later  becomes 
double,  and  then  the  two  plates  separate,  or  (c)  two  nuclear  mem- 
branes are  built  up  inside  of  the  old  membrane,  which  then  breaks 
down,  allowing  the  daughter  nuclei  to  escape. 

"  The  cell  body  is  the  last  to  divide  in  amitosis,  and  in  many, 
perhaps  the  majority  of  cases,  it  does  not  do  so  at  all"  (21,  p.  38). 


FIG.  7.    Amitotic  nuclear  division  in  the  follicle  cells  of  a  cricket's  egg.   (From 
Dahlgren  and  Kepner.) 

The  amitotic  process  is  usually  a  sign  of  senescence  or  decadence  in 
a  cell.  Many  cells  divide  by  mitosis  so  long  as  there  is  actual 
division  of  the  cytoplasm  as  well  as  of  the  nucleus,  but  if  the  cell 
body  fails  to  divide,  the  nucleus  alone  may  continue  to  multiply 
amitotically,  producing  a  multinucleate  cell.  "  The  commonly 
received  idea,  at  present,  concerning  amitosis  is  that  it  is  a  termi- 
nal process  in  the  cell's  life  activities,  and  is  a  method  of  securing 
more  nuclear  surface  for  use  in  forced  metabolism  or  secretion  " 

(21,  p.  39)- 

In  concluding  this  account  of  cell  division  two  points  are  worthy 
of  special  emphasis.  First,  with  regard  to  the  continuity  of  the 
chromatin,  it  may  be  said  that  the  chromatin  is  continuous 
from  one  cell  generation  to  another.  The  cells  resulting  from 
mitosis  may  differ  greatly  in  size,  but  the  chromatin  seems  to 
be  divided  equally  between  them  with  great  exactness.  Second, 
cells  are  never  known  to  arise  except  from  preexisting  cells.  These 
two  facts  are  perhaps  the  most  important  for  us  to  keep  in  mind 
as  we  go  on  to  study  the  more  complex  problems  of  fertilization 
and  cell  division  in  the  many-celled  animals,  for  growth  in  every 
Metazoon  is  really  nothing  more  than  cell  division  and  cell 
growth. 


34 


AN   INTRODUCTION   TO  ZOOLOGY 


3.  THE  CELL  THEORY 


Owing  to  the  important  place  that  cells  have  occupied  in  the 
development  of  our  fundamental  biological  ideas,  it  seems  proper 
to  consider  briefly  the  origin  and  growth  of  the  cell  theory.  Cells 
were  first  described  by  Hooke,  an  Englishman,  in  1665.  He 
published  a  figure  showing  the  minute  structure  of  cork  (Fig.  8). 


FIG.  8.     Facsimile  of  a  figure  by  Hooke  representing  cells  of  cork.     (From 
Farmer  in  Lankester's  Zoology.) 

The  regular  arrangement  of  the  compartments  in  this  tissue 
reminded  him  of  the  cells  of  the  monks  in  a  monastery.  For  this 
reason  they  were  given  the  name  "  cell,"  which  they  bear  to  this 
day.  Several  later  investigators  observed  cellular  structures  in 
the  tissues  of  plants  and  animals  without  realizing  their  impor- 
tance. 

In  1833,  Brown,  an  English  botanist,  described  the  nucleus  as  a 
constant  element  of  the  cell.  It  was  not,  however,  until  the  time 
of  Schleiden,  a  botanist,  and  Schwann,  a  zoologist,  that  the  cell 
theory  was  established.  Schleiden,  in  1838,  published  a  small 
pamphlet  in  which  he  advanced  the  idea  that  all  plants  are  com- 
posed of  cells.  A  year  later  his  colleague,  Schwann.  brought 
forth  a  more  pretentious  work  in  which  he  made  the  same  generali- 


THE   CELL  AND  THE   CELL  THEORY  35 

zation  concerning  animals.  It  was  largely  due  to  the  careful 
work  of  the  latter  that  the  cell  theory  was  widely  accepted 
throughout  the  scientific  world. 

The  cell  theory  gave  a  basis  for  a  comparison  between  plants 
and  animals,  as  well  as  a  new  point  of  view  in  the  study  of  the 
tissues  of  multicellular  organisms.  A  host  of  investigators  have 
entered  the  fields  of  histology  and  cytology;  in  fact,  these  are 
still  the  favorite  subjects  for  anatomists. 

The  early  investigators  believed  the  cell  wall  to  be  the  im- 
portant part  of  the  cell.  Cohn,  in  1847,  observed  that  the  proto- 
plasm of  certain  lower  plants  left  the  wall  at  certain  times  and 
swam  about  in  the  water.  This  discovery  initiated  the  idea  that 
the  wall  is  of  minor  importance  as  compared  with  the  substance 
contained  within  it.  It  was  later  ascertained  that  many  cells 
have  no  walls  at  all. 

Perhaps  the  foremost  of  recent  workers  was  Max  Schultze, 
who  recognized  the  cytoplasm  and  nucleus  as  the  principal  con- 
stituents of  all  cells,  and,  in  1861,  defined  a  cell  as  a  mass  of  proto- 
plasm containing  a  nucleus.  He  also  stated  that  both  the  nucleus 
and  cytoplasm  arise  through  the  division  of  the  corresponding 
elements  of  a  preexisting  cell.  At  the  present  time,  though  we 
retain  the  old  designation  "  cell  "  for  the  units  of  protoplasm, 
it  must  be  constantly  borne  in  mind  that  the  name  refers  to  the 
living  substance  rather  than  to  the  wall,  which  may  or  may  not 
be  present. 

A  more  fitting  close  for  this  chapter  cannot  be  found  than  the 
words  of  Professor  E.  B.  Wilson,  the  foremost  American  investi- 
gator of  cellular  phenomena. 

"  During  the  half  century  that  has  elapsed  since  the  enuncia- 
tion of  the  cell-theory  by  Schleiden  and  Schwann,  in  1838-1839, 
it  has  become  ever  more  clearly  apparent  that  the  key  to  all 
ultimate  biological  problems  must,  in  the  last  analysis,  be  sought 
in  the  cell.  It  was  the  cell-theory  that  first  brought  the  structure 
of  plants  and  animals  under  one  point  of  view,  by  revealing  their 
common  plan  of  organization.  It  was  through  the  cell-theory 


36  AN  INTRODUCTION  TO  ZOOLOGY 

that  Kolliker,  Remak,  Nageli,  and  Hofmeister  opened  the  way 
to  an  understanding  of  the  nature  of  embryological  develop- 
ment, and  the  law  of  genetic  continuity  lying  at  the  basis  of 
inheritance.  It  was  the  cell-theory  again  which,  in  the  hands 
of  Goodsir,  Virchow,  and  Max  Schultze,  inaugurated  a  new  era 
in  the  history  of  physiology  and  pathology,  by  showing  that  all 
the  various  functions  of  the  body,  in  health  and  in  disease,  are 
but  the  outward  expressions  of  cell  activities.  And  at  a  still 
later  day  it  was  through  the  cell-theory  that  Hertwig,  Fol,  Van 
Beneden,  and  Strasburger  solved  the  long-standing  riddle  of  the 
fertilization  of  the  egg  and  the  mechanism  of  hereditary  trans- 
mission. No  other  biological  generalization,  save  only  the  theory 
of  organic  evolution,  has  brought  so  many  apparently  diverse 
phenomena  under  a  common  point  of  view,  or  has  accomplished 
more  for  the  unification  of  knowledge.  The  cell-theory  must 
therefore  be  placed  beside  the  evolution-theory  as  one  of  the 
foundation  stones  of  modern  biology  "  (25,  p.  i). 


CHAPTER  IV 
AMEBA 

(Ameba  proteus  Leidy) 

Ameba  proteus  (Fig.  9)  is  a  one-celled  animal  about  .25  mm. 
(T^g-  inch)  in  diameter,  and  is,  therefore,  invisible  to  the  naked  eye. 
Under  the  compound  microscope  it  appears  as  an  irregular  color- 


FIG.  9.  Ameba  proteus .  i,  nucleus  ;  2,  contractile  vacuole  ;  j,  pseudopodia, 
dotted  line  leads  to  ectoplasm ;  4,  food  vacuoles ;  5,  grains  of  sand. 
(From  Shipley  and  MacBride  after  Gruber.) 

less  particle  of  animated  jelly  which  is  constantly  changing  its 
shape  by  thrusting  out  finger-like  processes  (Fig.  9,  j). 

37 


38  AN   INTRODUCTION  TO  ZOOLOGY 

Habitat  and  Collecting.  —  Ameba  proteus  lives  in  fresh- water 
ponds  and  streams..  It  may  be  obtained  for  laboratory  use  from 
a  variety  of  places,  such  as  the  organic  ooze  from  decaying  vege- 
tation or  the  lower  surface  of  lily  pads.  Perhaps  the  most  cer- 
tain method  is  that  suggested  by  Professor  Jennings  (37).  About 
two  weeks  before  the  specimens  are  needed,  a  mass  of  pond  weed 
(Ceratophyllum  is  the  best)  should  be  gathered,  placed  in  flat 
dishes,  and  immersed  in  water.  The  vegetation  soon  decays,  and 
a  brown  scum  appears  on  the  surface.  In  this  scum  Amebas 
may  be  found. 

General  Anatomy.  —  Two  regions  are  distinguishable  in  the 
body  of  Ameba,  an  outer  colorless  layer  of  clear  cytoplasm,  the 
ectosarc,  and  a  comparatively  large  central  mass  of  granular  cyto- 
plasm, the  endosarc.  A  single  clear  spherical  body,  usually  lying 
near  the  end  of  the  animal  away  from  the  direction  of  motion,  and 
disappearing  at  more  or  less  regular  intervals,  is  the  contractile  or 
pulsating  vacuole  (Fig.  9,  2).  Suspended  in  the  endosarc  is  a 
nucleus  (Fig.  9,  /),  usually  one  or  more  food  v'acuoles  (Fig.  9,  4), 
material  ready  for  excretion,  foreign  substances  such  as  grains  of 
sand  (Fig.  9,  5),  and  undigested  particles,  the  amount  of  the 
latter  depending  upon  the  feeding  activity  of  the  specimen  at  the 
time  when  examined. 

From  this  description  it  will  be  noted  that  Ameba  proteus 
contains  all  of  the  essential  constituents  of  a  cell.  It  is,  moreover, 
simple  in  structure,  shows  a  number  of  physiological  activities 
in  their  simplest  form,  is  one  of  the  most  primitive  of  all  animals, 
and  is  easily  obtained.  For  these  reasons  it  has  been,  and  still  is, 
a  favorite  subject  for  study. 

Detailed  Anatomy.  —  The  ectoplasm  (Fig.  9,  3)  is  easily  dis- 
tinguished from  the  endoplasm  because  of  the  absence  within  it 
of  granules.  The  ectoplasm  is  firmer  than  the  endoplasm,  prob- 
ably because  the  outer  protoplasm  tends  to  stiffen  under  the 
influence  of  surface  tension. 

The  endoplasm  occupies  the  central  portion  of  the  body.  Be- 
ing less  dense  than  the  ectoplasm,  it  contains  within  it  all  of  the 


AMEBA  39 

large  granules.  No  fixed  line  of  separation  between  it  and  the 
ectoplasm  is  possible. 

The  nucleus  (Fig.  9,  i)  is  not  easily  seen  in  living  specimens. 
In  animals  that  have  been  properly  killed  and  stained  it  appears  as 
a  spherical  body  lying  in  the  endoplasm.  Its  position  is  not 
definite,  but  changes  during  the  movements  of  the  Ameba.  It  has 
a  firm  membrane  and  contains  a  great  many  spherical  particles  of 
chromatin  scattered  about  in  the  nuclear  sap.  During  the  life 
of  Ameba,  before  the  period  of  reproduction,  the  nucleus  plays 
an  important  role  in  the  metabolic  activity  of  the  cell.  This  has 
been  proved  by  experiments  in  which  the  animal  was  cut  in  two. 
Invariably  the  vital  processes  were  disturbed  in  the  enucleated 
fragment,  and  death  resulted.  Profound  changes  in  the  nucleus 
take  place  during  reproduction;  these  will  be  described  in  detail 
later. 

The  contractile  vacuole  (Fig.  9,  2]  is  a  clear  space  filled  with  a 
fluid  less  dense  than  the  surrounding  protoplasm.  It  derives  its 
name  from  the  fact  that  at  more  or  less  regular  intervals  it  sud- 
denly disappears,  its  walls  having  contracted,  thus  forcing  out 
the  contents.  That  the  vacuole  discharges  to  the  outside  of  the 
body  has  not  been  definitely  observed  in  Ameba,  no  doubt  because 
the  fluid  is  usually  expelled  on  the  upper  surface  of  the  body  and 
therefore  cannot  be  seen  (38).  "  At  first  the  vacuole  lies  near 
the  nucleus,  but  as  it  grows,  it  becomes  separated  from  the  latter, 
and  at  the  time  of  its  contraction  lies  at  the  end  of  the  body  far- 
thest from  the  advancing  pseudopodia,  at  what  is  sometimes  called 
the  posterior  end.  Its  reappearance  is  always  somewhere  near 
its  point  of  disappearance.  While  still  small  it  is  carried  along 
by  the  streaming  protoplasm  back  to  a  position  near  the  nucleus, 
where  it  completes  its  development  "  (29,  p.  88,  Fig.  10). 

The  functions  of  the  contractile  vacuole  are  excretory  and  res- 
piratory, as  is  explained  on  page  50.  Several  investigators  have 
recently  regarded  it  as  a  hydrostatic  organ,  regulating  the  quantity 
of  water  contained  in  the  body  of  the  animal,  and  so  its  weight. 
This  would  also  afford  a  means  of  getting  rid  of  the  water  taken 


40  AN  INTRODUCTION  TO  ZOOLOGY 

•" 


H  1  J 

FIG.  10.  Ameba  proteus.  A,  B,  C,  D,  E,  F,  show  successive  stages  in  the 
growth  of  the  contractile  vacuole  (i>) ;  G,  H,  I,  and  J,  show  four  stages 
in  the  contraction  of  the  vacuole.  (From  Calkins.) 


AMEBA  41 

in  by  the  protoplasm  through  the  body  wall  and  consequently 
regulating  the  tension  between  the  protoplasm  and  the  surround- 
ing water.  Death  by  diffluence  is  thus  prevented. 

Food  vacuoles  (Fig.  9,  4)  are  produced  when  food  is  taken  into 
the  body.  Such  a  vacuole  has  as  its  nucleus  a  particle  of  nutri- 
tive material  and  is  a  sort  of  temporary  stomach. 

Locomotion.  —  Although  extremely  simple  in  structure,  Ameba 
carries  on  practically  all  of  the  vital  activities  characteristic  of 
the  higher  animals.  It  is  capable  of  automatic  movement,  of 
reacting  to  various  stimuli,  of  carrying  on  metabolic  processes, 
of  growth,  and  of  reproduction.  These  are  all  fundamental 
properties  of  protoplasm  (Chap.  II,  p.  10)  and  are  here  ex- 
hibited in  their  simplest  form. 

The  locomotive  function  is  located  in  the  ectoplasm.  The 
motile  organs  are  finger-like  protrusions  of  cytoplasm  called 
pseudo podia.  A  pseudopodium  is  formed  in  the  following  man- 
ner. The  ectoplasm  bulges  out  and  enlarges  until  a  blunt  pro- 
jection is  produced;  the  endoplasm  then  flows  into  it.  The 
result  is  a  movement  of  the  entire  animal  in  the  direction  of  the 
pseudopodium.  If  more  than  one  is  formed  at  the  same  time, 
there  occurs  a  struggle  for  supremacy  until  finally  one  survives 
while  the  others  flow  back  and  gradually  disappear.  Ameba 
moves,  therefore,  by  thrusting  out  pseudopodia  and  then  flowing 
into  them.  It  is  necessary  to  distinguish  between  pseudopodia 
that  adhere  to  the  surface  of  some  object  and  those  that  are 
thrust  out  freely  into  the  surrounding  fluid,  since  an  hypothesis 
that  explains  the  formation  of  one  does  not  always  account  for 
the  other. 

There  are  three  principal  theories  which  attempt  to  explain 
the  formation  of  pseudopodia;  these  may  be  outlined  briefly  as 
follows :  — 

(i)  The  Adherence  Theory.  —  It  was  long  ago  discovered  that 
if  a  drop  of  inorganic  fluid  which  is  spreading  on  a  solid  surface 
is  made  to  adhere  on  one  side  more  strongly  than  the  other,  it 
will  move  toward  the  former  side.  This  fact  suggested  that  the 


42  AN  INTRODUCTION  TO  ZOOLOGY 

body  of  the  Ameba  may  move  in  a  certain  direction  for  the  same 
reason  (27).  This  theory,  however,  accounts  only  for  the 
pseudopodia  that  are  attached,  and  does  not  explain  free  pseudo- 
podia. 

(2)  The  Surface  Tension  Theory.  —  According  to  the  surface 
tension  theory,  local  changes  in  the  surface  tension  produce 
currents  which  move  forward  in  the  central  axis  and  backward 
along  the  surface  (28,  44).     It  has  been  shown,  however,  that 
the  currents  do  not  run  in  this  manner  (34). 

(3)  The  Contractile  Theory.  —  Many  early  investigators  held 
the  view  that  the  contractility  of  the  protoplasm  accounts  for 
changes  which  take  place  in  a  moving  Ameba.     This  theory  was 
later  given  up  by  most  zoologists  because  the  currents  in  the  pro- 
toplasm began  at  the  point  of  advance  and  extended  backward 


FIG.  ii.     Diagram  of  the  movements  of  a  particle  attached  to  the  outer  sur- 
face of  Ameba  verrucosa,  in  side  view.     (From  Jennings.) 

instead  of  commencing  at  the  hinder  end  or  in  the  center.  Two 
recent  authors  have  again  brought  forth  evidence  which  tends 
to  show  that  after  all  the  contraction  theory  is  the  correct  inter- 
pretation. Jennings  (39)  by  mixing  soot  in  water  was  able  to 
show  that  Ameba  verrucosa  resembles  an  elastic  sac  filled  with  a 
fluid.  Specimens  were  placed  in  water  containing  some  fine  soot. 
The  particles  became  attached  to  the  surface  of  the  animal  and 
the  currents  in  the  ectoplasm  could  easily  be  determined  by 
watching  their  movements.  From  these  observations  the  follow- 
ing conclusions  were  reached.  "In  an  advancing  Amoeba  sub- 
stance flows  forward  on  the  upper  surface,  rolls  over  at  the  ante- 
rior edge,  coming  in  contact  with  the  substratum,  then  remains 
quiet  until  the  body  of  the  Amoeba  has  passed  over  it.  It  ther 


AMEBA 


43 


moves  upward  at  the  posterior  end,  and  forward  again  on  the 
upper  surface,  continuing  in  rotation  as  long  as  the  Amceba  con- 
tinues to  progress.  The  motion  of  the  upper  surface  is  congruent 
with  that  of  the  endosarc,  the  two  forming  a  single  stream" 
(39,  p.  148,  Fig.  n).  "  The  movement  can  be  imitated  roughly 
by  making  a  cylinder  of  cloth,  laying  it  flat  on  a  plane  surface, 
and  pulling  forward  the  anterior  edge  in  a  series  of  waves.  The 
entire  cylinder  then  rolls  forward  just  as  the  Amoeba  does  " 
(39,  p.  145).  Jennings's  observations  have  been  confirmed  for 
Ameba  verrucosa,  but  do  not  seem  to  explain  the  phenomena  in 
moving  specimens  of  Ameba  proteus  and  other  species  (34). 

A  new  method  of  making  observations  has  enabled  Bellinger 
(34)  to  add  materially  to  our  knowledge  of  the  movements  of 


Polished    Edge 


FIG.  12.     Diagram  of  apparatus  designed  to  study  Ameba  in  side  view. 
(From  Dellinger  in  Journ.  Exp.  Zool.) 

Ameba  proteus.  This  investigator  examined  specimens  from  the 
side  by  means  of  the  following  apparatus  (Fig.  12):  One  edge 
of  a  slide  was  ground  square  and  polished.  Long  cover  slips  were 
cemented  to  this  with  the  edges  extending  beyond  the  polished 
surface  so  as  to  form  a  narrow  trough.  With  the  microscope 
brought  to  a  horizontal  position,  specimens  pipetted  into  the 
trough  could  be  observed  in  side  view  as  they  moved  along  the 
edge  of  the  slide.  Observations  made  in  this  way  seemed  tc 
prove  that  Ameba  moves  by  means  of  a  contractile  substance. 


44 


AN  INTRODUCTION  TO  ZOOLOGY 


In  advancing  the  Amebcz  "  extend  the  anterior  end  free  in  the 
water  and  attach  it  at  or  near  the  tip  and  then  contract.     At  the 


FIG.  13.  Photographs  of  Ameba  proteus  inside  view  to  show  method  of  loco- 
motion. A,  B,  and  C  show  a  specimen  attached  at  two  points  a  and  b;  the 
pseudopod  projecting  from  one  end  bends  down  to  the  substratum  as 
shown  in  B  at  d ;  C  shows  two  more  attached  pseudopods  at  c  and  /, 
and  several  free  pseudopods  at  £. 


AMEBA 


45 


FIG.  13  —  continued.     D  and  E  show  the  extension  and  attachment  of  a  long 
thin  pseudopod.     (From  Bellinger  in  Journ.  Exp.  ZooL) 

same  time  the  posterior  end  is  contracting  and  the  substance 
thus  pushed  and  pulled  forward  goes  to  form  the  new  anterior 
end.  This  continues  as  long  as  the  Ameba  advances  (Fig.  13, 
A,  B,  C).  Often  the  anterior  end  is  pushed  along  the  substratum, 
but  no  attachments  form  except  at  definite  points. 

"  In  other  cases  the  anterior  end  is  lifted  free  and  then  curves 
down  to  the  substratum  and  attaches,  forming  a  long  loop.  The 
posterior  end  is  then  released,  and  the  substance  flows  over  to 
the  anterior  end.  At  the  same  time  another  anterior  end  is  ex- 
tended "  (34,  p.  351,  Fig.  13,  D,  E).  When  creeping  on  the  ceil- 
ing the  movements  are  the  same  as  when  creeping  on  the  floor. 
When  beginning  to  move,  pseudopodia  are  extended  in  all  direc- 
tions until  one  becomes  attached,  when  the  animal  advances  in 
the  direction  of  the  point  of  attachment.  That  a  contractile 
substance  is  present  in  Ameba  and  accounts  for  its  movements 
seems  to  be  proven  by  Bellinger's  observations. 

Many  attempts  have  been  made  to  imitate  the  movements  o] 


46  AN  INTRODUCTION  TO  ZOOLOGY 

Ameba  by  means  of  inorganic  substances,  in  an  endeavor  to  dis< 
cover  the  physical  or  chemical  nature  of  its  locomotion  (40). 
Even  when  an  apparently  reliable  imitation  is  produced,  we  can- 
not be  certain  that  the  forces  at  work  are  actually  those  which 
cause  the  movements  of  Ameba.  Practically  all  of  the  imitations 
thus  far  reported  depend  on  surface  tension. 

One  method  of  producing  ameboid  movements  is  as  follows 
(26).  A  large  drop  of  mercury  is  placed  in  a  flat-bottomed  watch 
glass  and  covered  with  10  per  cent  nitric  acid.  A  piece  of  po- 
tassium bichromate  when  placed  near  the  mercury  produces  a 
solution  which  causes  local  lowering  of  the  surface  tension  of 
the  drop,  and  results  in  the  formation  of  projections  and  move- 
ment of  the  mercury  in  various  directions. 

Metabolism.  —  The  various  metabolic  activities  of  animals 
were  discussed  in  Chapter  II,  pages  13  to  15.  In  Ameba  these 
processes  are  seen  in  their  simplest  form,  and  will,  therefore,  be 
considered  in  some  detail  in  spite  of  the  danger  of  repetition. 
The  entire  series  of  processes  connected  with  the  manufacture 
and  destruction  of  protoplasm  are  the  ingestion  of  food,  diges- 
tion, egestion,  absorption,  circulation,  assimilation,  dissimila- 
tion, secretion,  excretion,  and  respiration. 

Food. — The  food  of  Ameba  consists  of  very  small  aquatic 
plants  such  as  Oscillaria,  and  diatoms,  Protozoa,  bacteria,  and 
other  animal  and  vegetable  matter.  A  certain  amount  of  choice 
of  food  is  exercised,  or  its  body  would  become  overloaded  with 
particles  of  sand  and  other  indigestible  material  among  which  it 
lives.  Furthermore,  it  seems  to  evince  a  preference  for  diatoms 
which  one  would  think  too  large  for  easy  consumption.  This 
apparent  choice  of  food  may  be  due  to  ordinary  physical  laws  of 
fluids. 

Ingestion.  —  The  ingestion,  or  taking  in  of  food,  occurs  with- 
out the  aid  of  a  mouth.  Food  may  be  engulfed  at  any  point  on 
the  surface  of  the  body,  but  it  is  usually  taken  in  at  what  may 
be  called  the  temporary  anterior  end,  that  is,  the  part  of  the  body 
toward  the  direction  of  locomotion.  Jennings  describes  inges- 


AMEBA 


47 


tion  as  follows.  The  Ameba  flows  against  the  food  particle, 
which  does  not  adhere  but  tends  to  be  pushed  forward  away 
from  the  animal  (Fig.  14,  i).  That  part  of  the  body  directly 
back  of  the  food  ceases  its  forward  movements  while,  on  either 
side  and  above,  pseudopodia  are  extended  which  gradually  form 
a  concavity  in  which  the  food  lies  (Fig.  14,  2)  and  finally  bend 
around  the  particle  (Fig.  14,  j)  until  their  ends  meet  and  fuse 


^j&f»>.oj%y  ^*t3«g!g££& 

FIG.  14.  Amcba  ingesting  a  Euglena  cyst,     i,  2,  3,  4,  successive  stages  in  the 
process.     (From  Jennings.) 

(Fig.  14,  4).  A  small  amount  of  water  is  taken  in  with  the  food, 
so  that  there  is  formed  a  vacuole  whose  sides  were  formerly  the 
outside  of  the  body,  and  whose  contents  consist  of  a  particle  of 
nutritive  material  suspended  in  water.  The  whole  process  of 
food-taking  occupies  one  or  more  minutes,  depending  on  the 
character  of  the  food.  Ameba  is  not  always  successful  in  ac- 
complishing what  it  undertakes,  but  when  it  does  not  capture  its 
prey  at  once,  it  seems  to  show  a  persistence  usually  only  attributed 
to  higher  organisms.  No  doubt  the  reactions  in  food-taking 
depend  upon  both  mechanical  and  chemical  stimuli  (39). 

Imitations  of  the  engulfing  of  food  by  Ameba  have  been  de- 
vised, based  on  the  theory  that  ingestion  depends  on  the  physical 
adhesion  between  the  liquid  protoplasm  and  the  solid  food. 
Drops  of  water,  glycerin,  white  of  egg,  etc.,  will  draw  into  con- 


48 


AN    INTRODUCTION  TO  ZOOLOGY 


tact  and  engulf  solid  particles  of  various  kinds.  One  of  the  most 
ingenious  of  these  imitations  is  that  reported  by  Rhumbler  of 
the  ingestion  of  a  filament  of  Oscillaria  (44).  A  thread  of  shellac 
is  brought  into  contact  with  a  drop  of  chloroform  in  a  watch 
glass  of  water.  The  drop  adheres  to  the  filament,  lengthens 
along  it,  and,  because  of  its  tendency  to  again  become  spherical, 
succeeds  in  bending  the  now  softened  thread.  The  tension  of  the 
surface  film  gradually  draws  in  more  of  the  filament,  until  finally 


FIG.  15.    Ameba  vcrrucosa  devouring  a  filament  of  Oscillaria.    (From  Rhum- 
bler in  Archiv  f.  Entwick.-mech.) 

the  whole  thread  is  embedded  in  a  complicated  coil  within  the 
drop.     The  ingestion  of  Oscillaria  by  Ameba  is  quite  similar 

(Fig.  15). 

Choice  of  food  may  also  be  imitated  with  inorganic  substances. 
For  example,  a  drop  of  chloroform  in  a  watch  glass  of  water  will 
take  in  shellac,  paraffin  and  other  substances,  and  will  reject 
sand,  wood,  glass,  etc.  (40).  The  substances  accepted  are  those 
which  adhere  to  the  drop  of  chloroform.  Since  in  the  majority 
of  cases  food  particles  do  not  stick  to  the  surface  of  the 


AMEBA 


49 


Ameba,  we  cannot  explain  ingestion  by  means  of  these  physical 
imitations. 

Digestion.  —  Digestion  takes  place  without  the  aid  of  a  stomach. 
After  a  food  vacuole  has  become  embedded  in  the  endoplasm 
its  walls  pour  into  it  a  secretion  of  some  mineral  acid,  probably 
HC1.  "  It  is  probable  that  the  minute  particles  of  nucleopro- 
teids  that  are  constantly  arising  in  the  neighborhood  of  the 
nucleus  contain  certain  digestive  ferments  which  stimulate  the 
formation  of  the  mineral  acid  in  the  vicinity  of  the  gastric 
vacuole  "  (32,  p.  79).  The  digestive  fluid  seems  to  dissolve  only 
proteid  substances,  having  no  effect  upon  fats  and  carbohydrates. 
That  the  nucleus  plays  an  important  role  in  digestive  processes 
was  shown  by  Hofer  (36),  who  found  that  if  a  well-fed  Ameba  was 
cut  in  two,  the  non-nucleated  portion  was  unable  to  digest  food. 

Egestion.  —  Indigestible  particles  are  egested  at  any  point 
on  the  surface  of  the  Ameba,  there  being  no  special  opening  to 
the  exterior  for  this  waste  matter.  Usually  such  particles  are 
heavier  than  the  protoplasm,  and  as  the  animal  moves  forward 
they  lag  behind,  finally  passing  out  at  the  end  away  from  the  direc- 
tion of  movement;  that  is,  Ameba  flows  away, leaving  the  indi- 
gestible solids  behind. 

Assimilation.  —  The  peptones,  derived  from  the  digestion  of 
proteid  substances,  together  with  the  water  and  mineral  matter 
taken  in  when  the  gastric  vacuole  was  formed,  are  absorbed  by 
the  surrounding  protoplasm  and  pass  into  the  body  substance  of 
the  animal,  no  circulatory  system  being  present  so  far  as  we  know. 
These  particles  of  organic  and  inorganic  matter  are  then  assimi- 
lated ;  that  is,  they  are  rearranged  to  form  new  particles  of  liv- 
ing protoplasm,  which  are  deposited  among  the  previously  existing 
particles.  The  ability  to  thus  manufacture  protoplasm  from 
unorganized  matter,  it  will  be  remembered,  is  one  of  the  funda- 
mental proper  ties  of  living  substance  (p.  10). 

Dissimilation.  —  The  energy  for  the  work  done  by  Ameba  comes 
from  the  breaking  down  of  complex  molecules  by  oxidation  or 
tf  physiological  burning."  This  is  known  as  dissimilation  or 


50  AN  INTRODUCTION  TO  ZOOLOGY 

katabolism.  The  products  of  this  slow  combustion  are  the  energy 
of  movement,  heat,  and  residual  matter.  Ordinarily  this  con- 
sists of  solids  and  fluids,  mainly  water,  some  mineral  substances, 
urea  and  CO2.  Secretions,  excretions,  and  the  products  of  respi- 
ration are  included  in  this  list. 

Secretion.  —  We  have  already  noted  that  an  acid  is  poured 
into  the  gastric  vacuole  by  the  surrounding  protoplasm.  Such  a 
product  of  dissimilation,  which  is  of  use  in  the  economy  of  the 
animal,  is  known  as  a  secretion. 

Excretion.  —  Materials  representing  the  final  reduction  of 
substances  in  the  process  of  katabolism  are  called  excretions. 
These  are  deposited  either  within  or  outside  of  the  body.  A  large 
part  of  the  excretory  matter,  including  urea  and  CO2,  passes 
through  the  general  surface  of  the  body.  The  fluid  content  of 
the  contractile  vacuole  is  known  to  contain  urea,  therefore  this 
organ  is  excretory  in  function.  It  is  also  respiratory,  since  CO2 
probably  makes  its  way  to  the  exterior  by  way  of  this  organ. 
Oxygen  dissolved  in  water  is  taken  in  through  the  surface  of  the 
body.  This  gas  is  necessary  for  the  life  of  the  animal;  if  replaced 
by  hydrogen,  movements  cease  after  twenty-four  hours;  if  air  is 
then  introduced,  movements  begin  again;  if  not,  death  ensues  (42). 

Growth.  —  If  food  is  plentiful,  more  substance  is  added  to  the 
living  protoplasm  of  the  Ameba  than  is  used  up  in  its  various 
physical  activities.  The  result  is  an  increase  in  the  volume  of 
the  animal.  This  is  growth,  and,  as  in  all  other  living  organisms, 
growth  by  the  addition  of  new  particles  among  the  preexisting 
particles,  i.e.  growth  by  intussusception. 

Reproduction.  —  There  is,  however,  a  limit  with  regard  to  the 
size  that  may  be  attained  by  A  meba  proteus,  as  it  rarely  exceeds 
.25  mm.  (T-^g-  inch)  in  diameter.  When  this  limit  is  reached,  the 
animal  divides  into  two  parts.  Why  should  there  be  such  a 
limit  ?  The  following  explanation  is  given  by  Herbert  Spencer 
and  others.  The  volume  of  an  organism  varies  as  the  cube  of 
its  diameter,  the  surface  as  the  square.  Thus,  as  an  animal 
grows,  the  ratio  between  surface  and  volume  decreases;  and,  since 


AMEBA  51 

Ameba  takes  in  food,  gives  off  waste  material,  and  carries  on 
respiration  through  its  surface,  the  activities  of  the  cell  must 
decrease  with  increase  in  size  until  further  growth  is  impossible. 
The  solution  of  the  problem  is  the  division  of  the  animal  into 
two,  whereby  the  original  ratio  between  surface  and  volume  is 
restored.  Reproduction  by  binary  division,  therefore,  takes 


fi  ^«?  */ •jto-iix  X-^  V   .f^./iy*- 


FIG.  16.     Ameba  poly  podia  dividing  by  binary  fission.     (From  Parker  and 
Haswell  after  F.  E.  Schulze.) 

place  when  growth  is  no  longer  possible.  It  is  supposed  that  this 
division  is  inaugurated  through  some  unknown  change  in  the 
relations  between  the  nucleus  and  cytoplasm.  There  are  at 
least  two  kinds  of  reproduction  in  Ameba  proteus,but  neither  has 
ever  been  satisfactorily  worked  out  in  detail.  They  are  (i;  binary 
division  and  (2)  sporulation. 


AN  INTRODUCTION  TO  ZOOLOGY 


(i)  During  binary  division  the  nucleus  apparently  divides  ami- 
totically,  although  indications  of  mitosis  have  been  reported.  A 
definite  mitotic  figure  is  formed  in  certain  species  of  Ameba, 
e.g.  Ameba  binudeata  (Fig.  17).  While  the  nucleus  is  elongating, 
a  constriction  appears  around  the  middle  of  the  cell;  the  nucleus 
then  separates  into  two  apparently  equal  parts  which  move 
toward  the  ends  of  the  animal.  Meanwhile  the  constriction  of 

the  body  has  grown  deeper,  and  finally 

severs  the  connection  between  the  two 
ends,  and  two  daughter  Amebae  result 
(47,  Fig.  16). 

(2)  Scheel  (46)  seems  to  be  the  only 
person  who  has  ever  witnessed  sporulation 
in  Ameba  proteus.  The  whole  process 
lasted  from  two  and  one  half  to  three 
months.  The  pseudopodia  were  first 
drawn  in  and  the  animal  became  spher- 
ical. A  three-layered  cyst  was  then 
secreted.  The  Ameba  rotated  within  this 
for  several  days,  after  which  all  move- 
ments ceased.  The  nucleus  divided  until 
there  were  twenty  or  thirty  present, 
arranged  near  the  surface.  Continued 
division  resulted  in  an  increase  of  nuclei 
to  from  five  hundred  to  six  hundred. 
The  central  part  of  the  body  had 
none.  Cell  walls  now  appeared  at  the  periphery,  cutting 
off  the  nuclei,  each  with  a  small  amount  of  the  surrounding 
cytoplasm.  The  wall  of  the  cyst  became  soft,  broke,  and 
allowed  the  small  Amebae  to  escape.  Hundreds  of  these 
Amebulae,  or  pseudopodios pores,  as  they  are  sometimes  called, 
broke  out  at  one  time.  They  became  recognizable  as  Ameba 
proteus  in  from  two  and  a  half  to  three  weeks.  No  reason 
could  be  discovered  for  sporulation,  although  experiments  were 
conducted  in  which  specimens  were  starved,  were  given  an  excess 


FIG.  17.  Ameba  binu- 
deata beginning  to  di- 
vide. Both  nuclei  have 
formed  mitotic  figures. 
(From  Lang  after 
Schaudinn.) 


AMEBA  53 

of  food,  were  allowed  to  desiccate,  and  were  transferred  to  water 
from  different  localities;  none  of  these  resulted  in  encystment  and 
sporulation. 

The  development  of  Ameba  is  simply  a  matter  of  growth; 
both  the  spores  and  the  daughter  cells  resulting  from  binary  divi- 
sion become  full-grown  specimens  by  means  of  a  gradual  increase 
in  volume. 

Behavior  of  Ameba.  —  The  sum  total  of  all  the  various  move- 
ments of  an  animal  constitute  what  is  known  as  its  behavior. 
In  Ameba  these  movements  may  be  separated  into  those  con- 
nected with  locomotion  and  those  resulting  from  external  stimuli. 
We  have  already  given  an  account  of  the  locomotion  of  Ameba, 
and  so  shall  confine  ourselves  now  to  a  discussion  of  its  responses 
to  different  kinds  of  stimuli.  The  reactions  of  Ameba  to  stimuli 
have  been  grouped  by  Jennings  (39,  41)  into  positive,  negative, 
and  food- taking.  The  last  named  were  discussed  on  page  47. 
The  following  account,  then,  will  deal  with  positive  and  negative 
reactions  of  Ameba  proteus  to  external  stimulation. 

Definitions  of  Terms.  —  First,  it  will  be  well  to  define  a  few  of 
the  terms  used  in  describing  the  motor  responses  of  animals  to 
stimuli.  A  reaction  resulting  from  a  change  in  an  animal's 
environment,  e.g.  an  increase  in  the  intensity  of  the  light,  is 
known  as  "  tropism  "  or  "  taxis."  The  term  "  tropism  "  means 
"  to  turn  ";  it  is  used  instead  of  saying  that  an  animal  "  likes  " 
or  "  dislikes  "  certain  stimuli,  because  we  can  determine  by  ex- 
periment which  way  an  individual  will  turn  under  various  condi- 
tions, but  are  not  in  a  position  to  decide  whether  "  likes  "  or 
"  dislikes  "  enter  into  the  phenomena.  The  kind  of  stimulus 
employed  is  indicated  by  a  prefix.  The  principal  kinds  of  tro- 
pisms  are  as  follows:  — 

(1)  Thigmotropism  =  reaction  to  contact. 

(2)  Chemo tropism  =  reaction  to  a  chemical. 

(3)  Thermotropism  =  reaction  to  heat. 

(4)  Phototropism  =  reaction  to  light. 

(5)  Electrotropism  =  reaction  to  electric  current 


54 


AN  INTRODUCTION  TO  ZOOLOGY 


FIG.  18. 


Ameba  stimulated 
glass  rod. 


(6)  Geotropism  =  reaction  to  gravity. 

(7)  Chromotropism  =  reaction  to  color. 

(8)  Rheotropism  =  reaction  to  current. 

"  Taxis  "  is  often  employed  instead  of  "  tropism"  when  the  terms 
read  "  thigmotaxis,"  "  chemotaxis,"  etc.     If  the  animal  reacts 

by  a  movement  toward  the  stim- 
ulus, such  as  light,  it  is  said  to 
be  positively  phototropic,  photo- 
tactic,  etc.;  if  away  from  the 
stimulus,  negatively  phototropic 
or  phototactic,  etc.  Ameba  has 
been  found  to  respond  to  contact 
with  solids,  to  chemicals,  to  heat, 
to  light,  to  colors,  and  to  elec- 
tricity. After  describing  the  re- 
actions of  Ameba  to  each  of  the 
above-named  stimuli,  the  general 
significance  of  its  behavior  will  be 
considered. 

Thigmotropism.  —  If  a  moving 
Ameba  is  touched  at  any  point 
with  a  solid  object,  e.g.  a  glass 
rod,  the  part  affected  contracts 

and  moves  away.  If  the  anterior  edge  is  stimulated  in  this  way, 
the  part  touched  stops  and  contracts  while  a  new  pseudopodium 
is  pushed  out  in  some  other  place  and  the  animal  moves  away  in 
another  direction  (Fig.  19).  This  is  clearly  a  negative  reaction, 
and  the  animal  is  said  to  be  negatively  thigmotropic.  There 
are,  however,  certain  conditions  which  call  forth  positive  re- 
sponses to  contact  with  solids.  For  example,  if  the  Ameba  is 
floating  freely  in  the  water  and  a  pseudopodium  comes  in  contact 
with  the  substratum  (Fig.  20)  the  animal  moves  in  the  direction 
of  the  pseudopodium  stimulated  until  the  normal  creeping  posi- 
tion has  been  attained.  Contact  with  food  also  results  in  posi- 
tive reactions,  as  described  on  pages  47  and  48.  Ameba}  there- 


FIG.  19.  Ameba  moves  always 
from  mechanical  stimulus. 
(From  Jennings.) 


AMEBA 


55 


fore,  reacts  negatively  to  a  strong  mechanical  stimulus  and  posi- 
tively to  a  weak  one. 

Chemotropism.  —  Ameba  is  sensitive  to  changes  in  the  chemi- 
cal composition  of  the  water  surrounding  it.     If  a  chemical  is 


a  ~b  c 

FIG.  20.    Method  by  which  a  floating  Ameba  passes  to  a  solid. 
(From  Jennings.) 

brought  into  contact  with  one  side  by  means  of  a  fine  capillary 
glass  rod  or  tube,  the  part  affected  contracts  and  the  animal 
moves  off  in  some  other  direction  (Fig.  21).  "It  has  been  shown 
to  react  negatively  when  the  following 
substances  come  in  contact  with  one  side 
of  its  body:  methylene  blue,  methyl  green, 
sodium  chloride,  sodium  carbonate,  potas- 
sium nitrate,  potassium  hydroxide,  acetic 

acid,  hydrochloric  acid,  cane  sugar,  dis-   FlG;  2I-    Nesative 

r  tion  to  chemical  stimu- 

tillea  water,  tap  water,  and  water  trom 

other  cultures  than  that  in  which  the 
Amceba  under  experimentation  lives " 
(41,  p.  10). 

Thermotropism.  —  A  meba  gives  a  nega- 
tive response  if  locally  affected  by  heat. 
This  may  be  accomplished  by  bringing 
a  heated  needle  in  front  of  an  Ameba 
which  is  creeping  along  on  the  under  surface  of  a  cover  glass. 
When  this  is  done,  the  animal  moves  off  in  another  direction. 
The  degree  of  temperature  also  has  an  influence  upon  the  rate 
of  the  normal  movements.  Cold  and  excessive  heat  retard  its 
activities,  while  a  moderate  amount  of  heat  increases  the  move- 


lation  in  Ameba.  A 
little  methyl  green  dif- 
fuses against  the  ad- 
vancing end,  causing 
new  pseudopodia  to  be 
thrust  out,  and  a  change 
in  the  direction  of  move- 
ment. (From  Jennings.) 


AN   INTRODUCTION  TO  ZOOLOGY 


f 


.         ment.     Activity  ceases 
-* — ^L_      altogether  between  30° 
and  35°  C.,  and  the  ani- 
mal is  killed  by  coagulation  of  the 
protoplasm  if  the   temperature  is 
raised  to  40°  C. 

Phototropism.  —  If  a  strong  light 
strikes  Ameba  from  the  side,  it  will 
orient  itself  in  the  direction  of  the 
rays  and  move  away  from  the  source 
illumination    (Fig.   22).       Ordinary  white    light 
causes  an  Ameba  to  stop  moving  (35). 

Chromotropism.  —  The  effect  of  blue  light  upon 
Ameba  is  similar  to  that  of  ordinary  white  light. 
Red  light  apparently  has  no  effect  whatever  upon 
its  normal  activities,  while  other  colors  cause  a  vari- 
able amount  of  decrease  in  its  movement. 

Electrotropism.  —  If  an  electric  current  is  passed 
through  an  Ameba,  it  contracts  on  the  positive  or 
anode  side  of  the  body  and  moves  toward  the 
cathode  or  negative  pole. 

In  Ameba  there  are  no  organs  that  can  be  com- 
pared with  what  we  call  sense  organs  in  higher  organ- 
isms, and  we  must  attribute  its  reactions  to  stimuli 
to  that  fundamental  property  of  protoplasm  called 
irritability.     The  superficial  layer  of 
cytoplasm  receives  the  stimulus   and 
transfers  the  effects  to  some  other  part 
of  the  body;   thus  may  be  shown  the 
phenomenon  of  internal 


FIG.  22.  Reaction  of  Ameba  to  light.  The 
arrows  indicate  the  direction  of  the  light 
rays  and  the  numbers  the  successive  posi- 
tions assumed  by  the  animal.  The  Ameba 
always  moves  away  from  the  source  of 
light.  (From  Jennings  after  Davenport.) 


irritability  or  conductiv- 
ity. Perhaps  the  most 
primitive  method  in  the 
animal  kingdom  of  a  re- 
action to  stimuli  is  illns- 


AMEBA  57 

trated  by  the  response  of  a  definite  point  on  the  body  of 
Ameba  to  a  strong  mechanical  stimulus.  This  may  be  compared 
with  what  happens  when  a  mass  of  inorganic  material  is  acted 
upon  by  a  force;  here  a  reaction  is  produced  directly  at  the 
point  of  contact.  In  orienting  itself  in  the  direction  of  the  light 
rays,  Ameba  shows  a  response  to  a  continuous  stimulus  which 
affects  the  entire  body  (Fig.  22);  if  the  source  of  illumination 
is  changed,  the  animal  changes  its  orientation.  The  cause  of 
the  reaction,  as  in  the  above  instance,  seems  to  be  in  most 
cases  a  change  in  the  environment.  The  response  is  always  pro- 
duced by  a  stimulation  directly  preceding  it,  but  the  animal  soon 
becomes  acclimated  to  the  new  environment,  and  its  activities 
become  normal.  This  is  shown  when  it  is  transferred  from  one 
culture  to  another  (chemotropism)  or  when  white  light  is  thrown 
upon  it  (phototropism). 

The  reactions  of  Ameba  to  stimuli  are  of  undoubted  value  to 
the  individual  and  to  the  preservation  of  the  race,  for  in  every 
instance  the  negative  reaction  is  produced  by  injurious  agents 
such  as  strong  chemicals,  heat,  and  mechanical  impacts,  while 
positive  reactions  are  produced  by  beneficial  agents.  The  re- 
sponses, therefore,  in  the  former  cases  carry  the  animal  out  of 
danger,  in  the  latter,  toward  a  safer  haven. 

Ameba  is  of  fundamental  interest  to  animal  psychologists, 
since  it  represents  the  "  animal  mind  "  in  its  most  primitive 
form.  The  behavior  of  Ameba  in  the  absence  of  external  stimuli, 
for  example  when  it  is  suspended  freely  in  the  water,  shows  that 
some  of  its  activities  are  initiated  by  internal  causes.  Whether 
or  not  the  animal  is  in  any  degree  conscious  is  a  question  still 
unanswered.  If  Ameba  has  recognizable  sensations,  they  must 
be  infinitely  less  in  both  quality  and  quantity  than  in  higher 
organisms.  Furthermore,  it  is  unable  to  learn  from  the  few  kinds 
of  experiences  it  does  pass  through,  and  is  therefore  lacking  in 
memory  images.  One  case  in  which  an  Ameba  attempted  to 
capture  another  is  recorded  by  Jennings,  and  seems  to  point  to 
something  higher  than  mere  physical  and  chemical  attractions. 


58  AN  INTRODUCTION  TO   ZOOLOGY 

In  this  instance  a  large  animal  showed  decided  persistence  in  its 
endeavors  to  engulf  a  smaller  Ameba;  several  times  it  reversed 
its  course  and  continued  the  pursuit. 

A  review  of  the  facts  thus  far  obtained  seems  to  show  that 
factors  are  present  in  the  behavior  of  Ameba  "  comparable  to  the 
habits,  reflexes,  and  automatic  activities  of  higher  organisms  " 
(39,  p.  234),  and  "  if  Amoeba  were  a  large  animal,  so  as  to  come 
within  the  everyday  experience  of  human  beings,  its  behavior 
would  at  once  call  forth  the  attribution  to  it  of  states  of  pleasure 
and  pain,  of  hunger,  desire,  and  the  like,  on  precisely  the  same 
basis  as  we  attribute  these  things  to  the  dog"  (41,  p.  336). 


CHAPTER  V 
PARAMECIUM 

(Paramecium  caudatum  Ehrbg.) 

Paramecium,  like  Ameba,  is  a  unicellular  organism,  but  is 
further  advanced  in  the  scale  of  life.  It  also  is  found  in  fresh 
water  ponds  and  streams,  and  is  very  easily  obtained.  Cultures 
prepared  for  Ameba  will  in  most  cases  sooner  or  later  contain  a 
host  of  Paramecia. 

General  Anatomy.  —  If  a  drop  of  water  containing  Paramecia 
is  placed  on  a  slide,  the  animals  may  be  seen  with  the  naked  eye 
moving  rapidly  from  place  to  place.  Under  the  microscope  they 
appear  cigar-shaped  (Fig.  23).  A  closer  view  reveals  a  depression 
(o.g.)  extending  from  the  end  directed  forward  in  swimming, 
obliquely  backward  and  toward  the  right,  ending  just  posterior 
to  the  middle  of  the  animal.  This  is  the  so-called  oral  groove  or 
peristome.  The  mouth  (m.)  is  situated  near  the  end  of  the  oral 
groove.  It  opens  into  a  funnel-shaped  depression  called  the 
cy  to  pharynx  or  gullet  (g.),  which  passes  obliquely  downward  and 
posteriorly  into  the  endosarc  (en.).  The  oral  groove  gives  the 
animal  an  unsymmetrical  appearance.  Since  Paramecium  swims 
with  the  slender  but  blunt  end  foremost,  we  are  able  to  distinguish 
this  as  the  anterior  end.  The  opposite  end,  which  is  thicker  but 
more  pointed,  represents  the  posterior  end,  while  the  side  contain- 
ing the  oral  groove  may  be  designated  as  oral  or  ventral,  the  oppo- 
site side  aboral  or  dorsal.  The  motile  organs  are  fine  threadlike 
cilia  regularly  arranged  over  the  surface.  Two  layers  of  cyto- 
plasm are  visible,  as  in  Ameba,  an  outer  comparatively  thin  clear 
area,  the  ectosarc  (ec.)  and  a  central  granular  mass,  the  endosarc 
(en.).  Besides  these  a  distinct  pellicle  (p.)  or  cuticle  is  present 

59 


6o 


AN  INTRODUCTION  TO  ZOOLOGY 


outside  of  the  ectosarc. 
Lying  in  the  ectoplasm 
are  a  great  number  of 
minute  sacs,  the  trichocysts 
(tr.)}  which  discharge  long 
threads  to  the  exterior 
when  properly  stimulated. 
One  large  contractile  vacuole 
is  situated  near  either  end 
of  the  body,  close  to  the 
dorsal  surface,  while  a 
variable  number  of  food 
vacuoles  (/.».)  may  usually 
be  seen.  The  nuclei  are 
two  in  number,  a  large 
macronucleus  (ma.)  and  a 
smaller  micronucleus  (mi.) ; 
these  are  suspended  in  the 
ectoplasm  near  the  mouth 
opening.  The  anal  spot 
(an.)  can  be  observed  only 
when  solid  particles  are  dis- 
charged. It  is  situated 
just  behind  the  posterior 
end  of  the  oral  groove. 
FIG.  23.  Paramecium  viewed  from  the  oral  Detailed  Anatomy.  — 
surface.  L,  left  side;  R,  right  side.  —,,  ,  L,  /T,. 

The   endo  plasm    (rig.    23, 
an.,  anus ;    cc.,  ectosarc ;    en.,  endo-  r  ^ 

sarc;  f.v.,  food  vacuoles;  g.,  gullet;  en-)  of  Paramecium  occu- 
m.,  mouth ;  ma.,  macronucleus ;  mi.,  pies  the  central  part  of 
micronucleus ;  o.g.,  oral  groove ;  p.,  the  body.  It  is  supposed 
pellicle;  /r.,  trichocyst  layer.  The  to  be  alveolar  in  structure, 
arrows  show  the  direction  of  move-  , ,  <• ..  i  i  i 

,    ,      ,     ,  Most  of  the  larger  granules 

ment  of    the    food    vacuoles.     (From  e.    & . 

Jennings.)  contained    within    it    are 

shown   by    microchemical 
reactions  to  be  reserve  food  particles;  they  flow  from  place  to 


PARAMECIUM  6l 

place,  indicating  that  the  protoplasm  is  of  a  fluid  nature.  The 
ectoplasm  (ec.)  does  not  contain  any  of  the  large  granules  char- 
acteristic of  the  endoplasm,  since  its  density  prevents  their  en- 
trance. In  this  respect  the  two  kinds  of  cytoplasm  resemble 
the  ectoplasm  and  endoplasm  of  Ameba.  Outside  of  the  ecto- 
plasm is  a  delicate  elastic  membrane,  the  pellicle  (p.)  or  cuticle. 
If  a  drop  or  two  of  35  per  cent  alcohol  is  added  to  a  drop  of 
water  containing  Paramecia,  the  pellicle  will  be  raised  in  some 
specimens  in  the  form  of  a  blister.  Under  the  higher  powers  of 
the  microscope  the  pellicle  is  then  seen  to  be  made  up  of  a  great 
number  of  hexagonal  areas  produced  by  striations  on  the  surface 
(Fig.  24,  A).  These  striations  are  really  very  fine  grooves  (Fig. 
24,  B),  which  cross  one  another  obliquely. 

The  distribution  of  the  motile  organs,  the  cilia,  corresponds 
to  the  arrangement  of  the  striations  on  the  cuticle,  since  one 
cilium  projects  from  the  center  of  each  hexagonal  area  (Fig.  24, 
A).  These  thread-like  structures  occur  on  all  parts  of  the  body, 
those  at  the  posterior  end  being  slightly  longer  than  elsewhere 
(Fig.  23).  A  cilium  may  be  compared  to  a  very  fine  pseudo- 
podium  which  has  become  a  permanent  structure.  It  is  an  out- 
growth of  the  cell  protoplasm,  coming  from  a  basal  body  called  a 
microsome  (Fig.  24,  B,  B)  which  appears  to  arise  from  the  nucleus. 
The  structure  and  movements  of  cilia  have  been  studied  by  inves- 
tigators for  many  years,  and  the  following  theories  have  been 
proposed  to  explain  them:  (i)  Cilia  are  somewhat  stiff,  lifeless 
processes  attached  to  the  cell  and  moved  by  active  elements 
within  the  cell  body  (49).  (2)  Cilia  are  hollow  elastic  sheaths 
into  which  a  fluid  is  injected  and  withdrawn.  (3)  Cilia  consist  of 
a  complex  fluid  owing  its  contractility  "  to  the  presence  of  a 
filament  of  kinetic  granules,  placed  along  one  edge  of  the  cilium, 
the  contraction  of  this  thread  furnishing  the  power  of  the  cilium  " 
(52,  p.  49).  (4)  Cilia  are  fibrillar,  the  movements  being  due  to 
the  contraction  of  the  fibrils.  We  are  inclined  to  favor  the  last 
theory,  since  the  movements  can  only  be  adequately  explained 
by  the  presence  of  a  contractile  framework.  Fibrils  have  been 


62  AN  INTRODUCTION  TO  ZOOLOGY 

demonstrated  in  the  flagella  of  several  unicellular  animals,  a 
flagellum  being  a  large  cilium.  For  example,  in  a  Protozoon 
Euglena  (see  p.  85),  the  flagellum  consists  of  four  fibrils  which 
extend  its  entire  length.  These  are  twisted  about  one  another 
in  a  spiral  of  two  and  one  half  turns.  They  can  be  traced  into 
the  animal,  where  they  branch  out  into  a  system  of  rootlets 
(Fig.  25).  In  the  smaller  cilia  these  fibrils  have  not  been  seen, 


FIG.  25.     Structure  of  the  flagellum  of  Euglena. 
(From  Bellinger  in  Journ.  Morph.) 

but  are  probably  present;  at  least,  we  can  account  for  all  the 
movements  made  by  cilia  if  such  a  condition  exists  as  in  the  flagel- 
lum of  Euglena.  A  fusion  of  cilia  frequently  takes  place,  forming 
membranelles.  In  Paramecium  this  has  occurred  within  the 
mouth  cavity,  producing  the  undulating  membrane  (Fig.  29,  Mb). 
This  is  attached  to  the  dorsal  wall  of  the  mouth,  and  guides  the 
food  particles  that  are  swept  within  its  reach. 

Just  beneath  the  cilia,  embedded  in  the  cortical  layer  of  the 
ectoplasm,  is  a  uniform  layer  of  spindle-shaped  structures  y^\ o" 
mm.  in  length,  lying  with  their  long  axes  perpendicular  to  the 
surface  (Fig.  24,  A,  tr.,  Fig.  24,  B,  T).  These  are  trichocysts. 
Their  distribution  is  shown  in  surface  view  in  Figure  24,  A.  The)7 
appear  to  be  cavities  in  the  ectoplasm  filled  with  a  semi-liquid 
homogeneous  substance  which  is  very  refractive.  They  arise 
in  the  neighborhood  of  the  nucleus,  and  probably  from  it  (72). 
A  small  amount  of  osmic  or  acetic  acid,  when  added  to  a  drop  of 
water  containing  Paramecia,  causes  in  some  cases  the  discharge 
of  the  trichocysts  to  the  exterior  through  very  small  canals. 
This  explosion  is  due  to  the  pressure  derived  from  the  contrac- 
tion of  the  cortical  layer  of  the  ectoplasm.  After  the  explosion, 


§       * 


*       % 

$    , « 


t    » 


:   * 


FIG.  24.  Parts  of  Paramecia  showing  cilia  and  trichocysts.  A,  Surface  vie\v 
showing  striations,  cilia,  and  trichocysts  (tr.) ;  B,  part  of  a  cross  section 
showing  ridges  (L)  on  the  surface,  cilia  (C)  coming  from  microsomes 
(B),  trichocysts  cut  longitudinally  (T)  lying  in  the  ectosarc  (Co),  and 
trichocysts  (T1)  and  a  food  vacuole  (2V)  lying  in  the  endoplasm  (En). 
C,  trichocysts  in  various  stages  of  extrusion  ;  Tk,  body  of  the  structure  ; 
Tf,  the  hairlike  projection.  (A  from  Schuberg  ;  B,  C  from  Maier  in 
Archivf.  Protist.) 


PARAMECIUM 


the  trichocysts  appear  as  long  threads  which  have  been  extended 
to  about  eight  times  their  former  length  (Fig.  24,  C).  Tricho- 
cysts are  supposed  to  function  as  weapons  of  offense  and  defense. 
It  is  said  that  their  contents  are  discharged  with  considerable 
force  and  that  they  contain  a  poison  strong  enough  to  paralyze 
any  single-celled  animal.  The  only  evidence  we  have  that  the 
trichocysts  are  weapons  of  defense  is  furnished  when  Paramecium 
encounters  its  enemy  Didinium.  If  the  seizing  organ  of  this 
Protozoon  becomes  fastened  in  the  Paramecium,  a  great  number 
of  trichocysts  near  the  place  of  the  injury  are  discharged  (Fig.  26). 
These  produce  a  substance  which  be- 
comes jelly-like  on  entering  the  water; 
this  tends  to  force  the  two  animals  apart, 
and,  if  the  Paramecium 
is  a  large  one,  frequently 
it  succeeds  in  making 
its  escape  (70). 


FIG.  26.  Paramecium  defending  itself  from  an  attack  by  a  Protozoon 
Didinium.  The  trichocysts  are  discharged  and  mechanically  force  the 
enemy  away.  (From  Mast  in  Biol.  Bui.) 

Two  contractile  vacuoles  are  present,  occupying  definite  posi- 
tions, one  near  either  end  of  the  body.  They  lie  between  the 
ectoplasm  and  the  endoplasm,  close  to  the  dorsal  surface,  and  com- 
municate with  a  large  portion  of  the  body  by  means  of  a  system 
of  radiating  canals,  six  to  ten  in  number.  The  vacuoles  grow  in 
size  by  the  addition  of  liquid  which  is  excreted  by  the  protoplasm 
into  the  canals  and  is  then  poured  into  them.  When  the  full 


AN  INTRODUCTION  TO  ZOOLOGY 


size  is  reached,  the  walls  contract  and  the  contents  are  discharged 
to  the  exterior,  probably  through  a  pore.  The  two  vacuoles  do 
not  contract  at  the  same  time,  but  alternately,  the  interval  be- 
tween successive  contractions  being  ten  to  twenty  seconds.  The 
expulsion  of  the  fluid  contents  of  the  contractile  vacuoles  may  be 

seen  in  the  fol- 
lo  wing  way. 
Paramecia  should 
be  mounted  in 
water  into  which 
has  been  rubbed 
up  a  stick  of  India 
or  Chinese  ink. 
They  then  appear 
white  against  a 
black  back- 
ground. Part  of 
the  water  should 
be  withdrawn 

FIG.  27.  Paramecia  swimming  in  a  solution  of  India  Irom  beneath  me 

ink,   showing   the   discharge   of   the   contractile  cover   glass,   thus 

vacuoles  to  the  outside.     (From  Dahlgren   and  slightly  compress- 

Kepner  after  Jennings.)  ing  them      jf  now 

a  specimen  in  profile  is  found  and  watched,  the  discharge  pro- 
duces a  bright  spot  outside  in  the  opaque  liquid;  this  lasts  from 
one  to  two  seconds,  and  is  then  driven  off  by  the  cilia  (60,  Fig. 
27).  What  has  been  said  of  the  function  of  the  contractile 
vacuole  in  Ameba  (p.  39)  applies  as  well  to  those  of  Parame- 
cium, i.e.  it  acts  as  an  organ  of  excretion  and  respiration,  and  is 
probably  hydrostatic  (50). 

Locomotion.  —  The  only  movements  of  Paramecium  that  in 
any  way  resemble  those  of  Ameba  are  seen  when  the  animal  passes 
through  a  space  smaller  than  its  shorter  diameter;  it  will  then 
exhibit  an  elasticity  which  allows  it  to  squirm  through.  In  a 
free  field  Paramecium  swims  by  means  of  its  cilia.  "  These  are 


PARAMECIUM 


1 


usually  inclined  backward,  and  their  stroke  then  drives  the  animal 
forward.  They  may  at  times  be  directed  forward;  their  stroke 
then  drives  the  animal  backward. 
The  direction  of  their  effective 
stroke  may  indeed  be  varied  in 
many  ways,  as  we  shall  see  later. 
The  stroke  of  the  cilia  is  always 
somewhat  oblique,  so  that  in  addi- 
tion to  its  forward  or  backward 
movement  Paramecium  rotates  on 
its  long  axis.  This  rotation  is  over 
to  the  left,  both  when  the  animal 
is  swimming  forward  and  when  it 
is  swimming  backward.  The  revo- 
lution on  the  long  axis  is  not  due 
to  the  oblique  position  of  the  oral 
groove,  as  might  be  supposed,  for  if 
the  animal  is  cut  in  two,  the  pos- 
terior half,  which  has  no  oral  groove, 
continues  to  revolve. 

"The  cilia  in  the  oral  groove 
beat  more  effectively  than  those 
elsewhere.  The  result  is  to  turn 
the  anterior  end  continually  away 
from  the  oral  side,  just  as  happens 
in  a  boat  that  is  rowed  on  one  side 
more  strongly  than  on  the  other. 
As  a  result  the  animal  would  swim 
in  circles,  turning  continually  to- 
ward the  aboral  side,  but  for  the 
fact  that  it  rotates  on  its  long  axis. 
Through  the  rotation  the  forward 
movement  and  the  swerving  to  one 
side  are  combined  to  produce  a  spi- 
ral course.  The  swerving  when  the 


66  AN  INTRODUCTION  TO  ZOOLOGY 

oral  side  is  to  the  left,  is  to  the  right;  when  the  oral  side  is  above, 
the  body  swerves  downward ;  when  the  oral  side  is  to  the  right, 
the  body  swerves  to  the  left,  etc.  Hence  the  swerving  in  any 
given  direction  is  compensated  by  an  equal  swerving  in  the 
opposite  direction;  the  resultant  is  a  spiral  path  having  a  straight 
axis"  (62,  p.  44;  Fig.  28). 

Rotation  is  thus  effective  in  enabling  an  unsymmetrical  animal 
to  swim  in  a  straight  course  through  a  medium  which  allows 
deviations  to  right  or  left,  and  up  or  down.  It  is  well  known  that 
a  human  being  cannot  keep  a  straight  course  when  lost  in  the 
woods,  although  he  has  a  chance  to  err  only  to  the  right  or  left. 

Nutrition.  —  The  food  of  Paramecium  consists  principally  of 
bacteria  and  minute  Protozoa.  The  animal  does  not  wait  for 
the  food  to  come  within  its  reach,  but  by  continually  swimming 
from  place  to  place  is  able  to  enter  regions  where  favorable  food 
conditions  prevail.  The  cilia  also  aid  in  bringing  in  food  particles, 
since  a  sort  of  vortex  is  formed  by  their  arrangement  about  the 
oral  groove  which  directs  a  steady  stream  of  water  toward  the 
mouth. 

Figure  29  illustrates  the  formation  of  a  food  vacuole  (Ne). 
Food  particles  that  are  swept  into  the  mouth  (Mu)  are  carried 
down  into  the  cytopharynx  (s)  by  the  undulating  membrane 
(Mb) ;  they  are  then  moved  onward  by  the  cilia  lining  the  cyto- 
pharynx and  are  finally  gathered  together  at  the  end  of  the 
passageway  into  a  vacuole  which  gradually  forms  in  the  endo- 
plasm  ( Ne).  When  this  vacuole  has  reached  a  certain  size,  it  is 
pinched  off  from  the  extremity  of  the  cytopharynx  by  a  contrac- 
tion of  the  surrounding  protoplasm,  and  the  formation  of  another 
vacuole  is  begun.  A  food  vacuole  ( N)  is  a  droplet  of  water  with 
food  particles  suspended  within  it.  As  soon  as  one  is  separated 
from  the  cytopharynx,  it  is  swept  away  by  the  rotary  streaming 
movement  of  the  endoplasm  known  as  cyclosis.  This  carries 
the  food  vacuole  around  a  definite  course  which  begins  just  above 
and  behind  the  cytopharynx,  passes  backward  to  the  posterior 
end,  then  forward  near  the  dorsal  surface  to  the  anterior  end, 


-7 


FIG.  29.  A  section  from  the  region  of  the  mouth  of  Paramecium  showing 
the  formation  of  a  food  vacuole.  En.,  endosarc  ;  Ma.,  macronucleus  ; 
Mb.,  undulating  membrane  ;  Mu.,  mouth  ;  N.,  food  vacuole  ;  Ne.,  food 
vacuole  forming ;  S.,  gullet.  (From  Maier  in  Archiv  f.  Protist.) 


FIG.  30.  Binary  fission  of  Paramecium  aurelia.  i,  3,  4,  5,  contractile  vacu- 
oles  ;  2,  6,  dividing  macronucleus  ;  7,  9,  gullet ;  8,  10,  dividing  micro- 
nuclei.  (From  Lang's  Lehrbuch.) 


ol 

s 

U 


PARAMECIUM  67 

and  finally  downward  and  along  the  ventral  surface  toward  the 
mouth  (indicated  by  arrows  in  Fig.  23).  During  this  journey 
digestion  takes  place. 

Unlike  Ameba  a  special  anal  spot  (Fig.  23,  an.)  is  present  in 
Paramecium  through  which  indigestible  solids  are  discharged  to 
the  outside.  This  opens  on  the  ventral  surface  just  behind  the 
mouth.  It  can  be  seen  only  when  material  is  cast  out.  It  is  not 
yet  known  whether  the  anal  spot  is  a  permanent  orifice  whose 
lips  are  so  tightly  closed  as  to  be  invisible  to  us  or  whether  a  fresh 
opening  is  made  at  each  discharge.  The  processes  of  digestion, 
absorption,  dissimilation,  excretion,  respiration,  and  growth  are 
so  similar  to  those  described  for  Ameba  that  they  need  not  be 
considered  further  at  this  place  (see  pp.  49-50). 

Reproduction.  —  Paramecium  reproduces  only  by  simple 
binary  division.  This  process  is  interrupted  occasionally  by  a 
temporary  union  (conjugation)  of  two  individuals  and  a  subse- 
quent mutual  fertilization. 

Binary  fission.  —  In  binary  fission  the  animal  divides  trans- 
versely (Fig.  30).  The  first  indication  of  a  forthcoming  division 
is  seen  in  the  micronucleus,  which  undergoes  a  sort  of  mitosis 
(Fig.  30,  8  and  ici),  its  substance  being  equally  divided  between 
the  two  daughter  nuclei;  these  separate  and  finally  come  to  lie 
one  near  either  end  of  the  body.  Figure  30  shows  two  dividing 
micronuclei,  since  there  are  two  of  these  in  Paramecium  aurelia. 
The  macronucleus  elongates  and  then  divides  transversely 
(Fig.  30,  6).  The  gullet  produces  a  bud  which  develops  into 
another  gullet;  these  two  structures  move  apart,  the  old  gullet 
advancing  to  the  ventral  middle  line  of  the  forepart  of  the  body, 
and  the  new  one  to  a  similar  position  in  the  posterior  half 
(Fig.  30,  7  and  9).  The  undulating  membrane  remains  with  the 
old  gullet  while  a  new  one  arises  in  connection  with  the  new  gullet. 

new  contractile  vacuole  (Fig.  30,  i)  arises  near  the  anterior  end 
of  the  body,  another  just  back  of  the  middle  line  (Fig.  30,  4). 
While  these  events  are  taking  place  a  constriction  appears  near 
the  middle  of  the  longitudinal  diameter  of  the  body;  this  cleavage 


68  AN  INTRODUCTION  TO  ZOOLOGY 

furrow  becomes  deeper  and  deeper  until  only  a  slender  thread  of 
protoplasm  holds  the  two  halves  of  the  body  together.  This 
connection  is  finally  severed  and  the  two  daughter  Paramecia 
are  freed  from  each  other.  Each  contains  both  macro-  and  micro- 
nuclei,  two  contractile  vacuoles,  and  a  mouth  with  gullet.  The 
entire  process  occupies  about  two  hours.  The  time,  however, 
varies  considerably,  depending  upon  the  temperature  of  the  water, 
the  quality  and  quantity  of  food,  and  probably  other  factors. 
The  daughter  Paramecia  increase  rapidly  in  size,  and  at  the  end  of 
twenty-four  hours  divide  again  if  the  temperature  remains  at 
from  15°  to  17°  C. ;  if  the  temperature  is  raised  to  i7°-2o°  C., 
two  divisions  may  take  place  in  one  day  (83). 

Conjugation. — At  a  certain  time  in  the  life  cycle  of  Paramecium 
conjugation  occurs.  The  conditions  that  initiate  this  process  are 
not  yet  known,  but  the  complicated  stages  have  been  quite  fully 
worked  out.  When  two  Paramecia,  which  are  ready  to  conjugate, 
come  together,  they  remain  attached  to  each  other  because  of  the 
adhesive  state  of  the  external  protoplasm.  The  ventral  surfaces 
of  the  two  animals  are  opposed,  and  a  protoplasmic  bridge  is 
constructed  between  them.  As  soon  as  this  union  is  effected,  the 
nuclei  pass  through  a  series  of  stages  which  have  been  likened  to 
the  maturation  processes  of  metazoan  eggs  (Chap.  VII,  p.  103). 
Reference  to  Figure  31  will  help  to  make  clear  the  following  de- 
scription. The  micronucleus  moves  from  its  normal  position  in 
a  concavity  of  the  macronucleus  (Fig.  23,  mi.),  and  grows  larger, 
its  chromatin  breaking  up  into  granules  which  radiate  from  a 
division  center  at  one  end  (Fig.  31,0).  The  nucleus  then  length- 
ens, forming  a  spindle,  and  subsequently  divides  into  two  (b). 
These  immediately  divide  again  without  the  intervention  of  a 
resting  stage.  The  resultant  four  nuclei  (c)  have  been  compared 
to  the  four  sperms  produced  by  a  primary  spermatocyte  or  to  an 
egg  with  its  polar  bodies,  and  the  divisions  are  considered  as  the 
first  and  second  maturation  mitoses  (see  pp.  103-108).  Three 
of  the  four  nuclei  degenerate  (d),  the  fourth  divides  again. 
During  this  division  there  are  no  definite  spindle  fibers  and  no 


PARAMECIUM 


69 


longitudinal  splitting  of  the  chromosomes,  but  the  granules  of 
chromatin  contained  in  the  nuclei  separate  into  two  groups,  one 
smaller  (Fig.  32,  A,  m.  n.)  than  the  other  (Fig.  32,  A,  /.  ».). 


FIG.  31.  Diagram  showing  the  stages  in  the  conjugation  of  two  Paramecia 
and  the  subsequent  divisions  of  the  conjugants  during  the  period  of 
nuclear  reconstruction.  For  description  see  text.  (After  Maupas.) 


AN  INTRODUCTION  TO  ZOOLOGY 


/.n. 


m.n. 


FIG.  32.  Two  views  of  the  micronuclei  during  the  conjugation  of  Paramecium. 
A,  the  spindle  formed  during  the  division  of  the  micronucleus  which 
results  in  the  production  of  a  large  female  nucleus  (f.  n.)  and  a  smaller 
male  nucleus  (m.  n.)  ;  B,  the  fusion  of  the  male  nucleus  (m.  n.}  of  one 
animal  with  the  female  nucleus  (/.  n.}  of  the  other  conjugant.  (From 
Calkins  and  Cull,  in  Archiv  f.  Protist.) 


i 


PARAMECIUM  77 

These  groups  of  chromatic  material  then  become  recognizable 
as  distinct  nuclei  (Fig.  31,  e).  The  smaller  nucleus  might  be 
considered  comparable  to  the  male  nucleus,  the  other  the  female. 
The  male  nucleus  migrates  across  the  protoplasmic  bridge  between 
the  two  animals  (Fig.  31,7)  and  unites  with  the  female  nucleus  of 
the  other  conjugant  (Fig.  31,  g;  Fig.  32,  B),  forming  a  fusion 
nucleus  (h).  Thus  is  fertilization  effected. 

The  conjugants  separate  soon  after  fertilization  (Fig.  31,  g). 
The  macronucleus,  which  up  to  this  time  has  remained  at  rest, 
now  assumes  a  vermiform  shape,  breaks  up  into  small  segments, 
and  then  dissolves.  The  fusion  nucleus  of  each  conjugant, 
shortly  after  separation,  divides  by  mitosis  into  two  (i),  these  two 
into  four  (j)  and  these  four  into  eight  nuclei  equal  in  size  (k). 
Four  of  these  forming  a  group  near  the  anterior  end  increase  in 
size  and  develop  into  macronuclei  (/) ;  the  other  four  are  grouped 
near  the  posterior  end.  Three  of  these  degenerate,  the  fourth 
remains  a  micronudeus  (/).  The  animal  at  this  time  then  con- 
tains four  macronuclei  and  one  micronucleus  (m) .  This  micronu- 
cleus  divides  into  two  (n).  The  whole  animal  then  divides  by 
binary  fission,  each  daughter  cell  securing  two  of  the  macronuclei 
and  one  micronucleus  (0).  The  micronuclei  of  the  two  daughter 
cells  divide  again  (p) ,  and  another  binary  division  results  in  four 
cells  each  with  one  macronucleus  and  one  micronucleus  (q).  An 
indefinite  number  of  generations  are  produced  by  the  transverse 
division  of  the  four  daughter  cells  resulting  from  each  conjugant 

(71,  57,  54). 

Life  Cycle  (53). — Enough  is  known  of  the  life  history  of 
Paramecium  to  enable  us  to  give  a  brief  sketch  of  its  life  cycle. 
We  must  first  define  what  we  mean  by  an  individual.  It  has  been 
pointed  out  that  the  entire  series  of  cells  which  are  produced  by 
binary  division  from  the  time  of  one  conjugation  to  that  of  the 
next  ought  to  be  compared  with  a  many-celled  animal,  a  Meta- 
zoon  (54).  We  have  in  another  place  (p.  14)  shown  that  animals 
pass  through  three  phases  of  physical  activity,  youth,  maturity, 
and  old  age.  To  the  period  of  youth  we  attribute  a  high  degree 


72  AN  INTRODUCTION  TO  ZOOLOGY 

of  vitality,  rapid  cell  multiplication  and  growth,  and  active 
functions ;  at  maturity,  the  cell  division  becomes  less  rapid, 
growth  ends,  and  the  organism  becomes  sexually  mature;  in  old 
age,  cells  break  down,  functions  become  imperfect,  and  degenera- 
tion sets  in,  ending  in  natural  death.  These  three  stages  are  found 
in  the  life  cycle  of  Paramecium,  provided  we  accept  the  above 
definition  of  the  protozoan  individual;  youth  is  characterized 
by  rapidly  dividing  cells;  maturity  by  the  attainment  of  full 
size  and  conjugation;  and  old  age  by  degeneration  and  natural 
death  if  conjugation  is  prevented. 

In  the  stage  of  youth  Paramecia  do  not  conjugate  even  if  many 
are  confined  in  a  limited  space.  The  animals  divide  very  rapidly 
at  this  time.  They  are  usually  almost  transparent  and  free 
from  reserve  material  of  all  kinds,  and  are  able  to  resist  adverse 
conditions,  showing  a  high  grade  of  vitality;  this  is  due  to  the 
excess  of  constructive  over  destructive  metabolism.  There  is 
no  definite  limit  to  youth.  Maturity  comes  on  imperceptibly, 
and  mature  animals  can  be  recognized  only  when  in  some  phase 
of  sexual  activity. 

In  a  culture  under  continual  observation  a  decline  in  the  rate 
of  reproduction  indicates  the  approach  of  maturity.  The  proto- 
plasm at  this  stage  undergoes  a  change  both  physically  and 
chemically;  the  surface  layer  becomes  sticky,  so  that  when  two 
cells  meet  they  fuse,  and  conjugation  results.  This  frequently 
occurs  in  a  large  number  of  animals  in  a  single  culture  at  the  same 
time  and  a  so-called  "  epidemic  "  of  conjugation  may  then  be 
observed.  The  conjugants  are  smaller  than  the  other  specimens, 
being  only  .21  mm.  long,  while  the  usual  length  is  about  .3  mm. 
The  immediate  result  of  conjugation  is  apparently  the  rejuvena- 
tion of  the  conjugants. 

If  the  Paramecia  are  kept  in  a  constant  medium,  e.g.  "  hay  in- 
fusion," they  undergo  a  period  of  physiological  depression  about 
every  three  months,  as  shown  by  the  decrease  in  their  rate  of 
division.  These  periods  of  depression  are  due  to  unknown  meta- 
bolic conditions,  which  lessen  the  rapidity  of  division,  but  do  not 


PARAMECIUM  73 

cause  the  death  of  the  animal.  Semiannual  periods  also  occur, 
but  recovery  from  these  does  not  take  place  if  the  animals  are 
kept  under  constant  conditions  or  conjugation  is  prevented,  but 
the  protoplasm  degenerates  and  becomes  vacuolated  and  the  ani- 
mals lose  their  energies  and  finally  die. 

Experiments  have  been  performed  which  seem  to  show  that 
in  a  varied  environment  neither  conjugation  nor  death  from  old 
age  necessarily  occurs.  Thus  Woodruff  (1909,  1910)  has  carried 
a  culture  of  Paramecia  through  a  period  of  thirty-seven  months 
by  changing  the  character  of  the  medium  daily.  During  this 
time  there  were  seventeen  hundred  and  ninety-five  generations. 
At  the  end  of  the  thirty-seventh  month,  the  animals  were  normal. 
The  cycle  may  thus  be  prolonged  from  six  months  to  over  thirty- 
seven  months  by  employing  a  varied  culture  medium.  Since  in 
nature  the  stimuli  derived  from  changes  in  the  environment 
probably  are  present,  the  length  of  the  cycle  may  perhaps  be 
prolonged  indefinitely. 

Behavior.  —  Paramecium  is  a  more  active  animal  than  Ameba, 
swimming  across  the  field  of  the  microscope  so  rapidly  that  care- 
ful observations  are  necessary  to  discover  the  details  of  its  move- 
ments. As  in  Ameba,  its  activities  are  either  spontaneous,  that  is, 
initiated  because  of  some  internal  influence,  or  result  from  some 
external  stimulus.  This  stimulus  is  in  all  cases  a  change  in  the 
environment.  For  example,  if  a  drop  of  distilled  water  is  added  to 
a  drop  of  ordinary  culture  water  containing  a  number  of  Para- 
mecia, all  of  the  animals  will  enter  and  remain  in  the  distilled 
water;  they  are  stimulated  to  a  certain  kind  of  activity  by  the 
change  in  the  composition  of  the  water.  They  will  soon  become 
acclimated  to  their  new  surroundings,  and  will  behave  themselves 
within  the  distilled  water  in  a  normal  manner  until  another  change 
in  their  environment  stimulates  them  to  further  reactions. 

Paramecium  responds  to  stimuli  either  negatively  or  positively. 
The  negative  response  is  known  as  the  "avoiding  reaction"; 
it  takes  place  in  the  following  manner.  When  a  Paramecium 
receives  an  injurious  stimulus  at  its  anterior  end,  it  reverses  its 


74  AN  INTRODUCTION  TO  ZOOLOGY 

cilia  and  swims  backward  for  a  short  distance  out  of  the  region  oi 
stimulation;  then  its  rotation  decreases  in  rapidity  and  it  swerves 
toward  the  aboral  side  more  strongly  than  under  normal  condi- 
tions. Its  posterior  end  then  becomes  a  sort  of  pivot  upon  which 
the  animal  swings  about  in  a  circle  (Fig.  33).  During  this  revo- 
lution samples  of  the  surrounding  medium  are  brought  into  the 
oral  groove.  When  a  sample  no  longer  contains  the  stimulus, 
the  cilia  resume  their  normal  beating  and  the  animal  moves 


I.  *- 


c?. 


FIG.  33.  Diagram  of  the  avoiding  reaction  of  Parameclum.  A  is  a  solid 
object  or  other  source  of  stimulation.  1-6,  successive  positions  occupied 
by  the  animal.  (The  rotation  on  the  long  axis  is  not  shown.)  (From 
Jennings.) 

forward  again.  If  this  once  more  brings  it  into  the  region  of  the 
stimulus,  the  avoiding  reaction  is  repeated;  this  goes  on  as  long 
as  the  animal  receives  the  stimulus.  The  repetition  of  the  avoid- 
ing reaction  is  very  well  shown  when  Parameclum  enters  a  drop 
of  ^V  Per  cent  acetic  acid.  In  attempting  to  get  out  of  the  drop 
the  surrounding  water  is  encountered;  to  this  the  avoiding  reac- 
tion is  given  and  a  new  direction  is  taken  within  the  acid,  which 
of  course  leads  to  the  water  and  another  negative  reaction.  The 
accompanying  Figure  34  shows  part  of  the  pathway  made  by  a 
single  Paramecium  under  these  conditions. 

Positive  Reaction.  —  If  a  little  acid  is  placed  in  the  center  of  a 
large  drop  of  water   containing  a   number  of  Paramecia,  all  of 


PARAMECIUM 


75 


the  animals  in  the  drop  will  sooner  or  later  encounter  the  acid, 
and  having  once  entered  are  unable  to  escape,  just  as  in  the  case 
described  above.  A  group  is  therefore  formed  in  the  acid,  illus- 
trating what  is  called  a  positive 
reaction  and  in  this  case  positive 
chemotropism.  This  experiment 


a 


B 


.Q-.|Q1. 


o 

f  0 

•'.•:'; 
•''<" 

I 


FIG.  35.  Diagrams  showing  Paramecia 
collected  about  a  bubble  of  CO2  in 
the  optimum  concentration ;  a  is  a 
bubble  of  air,  b  of  CO2.  A  shows 
the  preparation  two  minutes  after 
the  introduction  of  the  CO2 ;  B,  two 
minutes  later ;  C,  eighteen  minutes 
later.  (From  Jennings.) 


ny  cases,  as  above,  the  passage 
weak  solution  causes  no  reaction. 


FIG.  34.  Path  followed  by  a  sin- 
gle Paramecium  in  a  drop  of 
acid.  (From  Jennings.) 

may  be  repeated  using  a  J 
per  cent  solution  of  common 
salt  in  which  are  placed  a 
number  of  specimens.  If  a 
drop  of  TV  per  cent  solution 
of  the  same  chemical  is  now 
added,  the  Paramecia  will 
swim  into  and  directly  across 
it,  but  on  reaching  the  bound- 
ary between  the  two  solutions 
on  the  other  side  of  the  drop, 
the  avoiding  reaction  will  be 
given.  Soon  the  weaker  so- 
lution will  contain  all  of  the 
animals  which,  having  once 
entered,  cannot  escape.  In 
from  a  strong  solution  to  a 
For  certain  substances,  how- 


76 


AN  INTRODUCTION  TO  ZOOLOGY 


ever,  there  is  a  definite  strength  which  seems  to  suit  the  Para- 
mecium  better  than  any  other,  and  no  reaction  takes  place  on 
entering  it.  Passage  from  such  a  solution  to  either  a  weaker  or 
a  stronger  calls  forth  the  avoiding  reaction.  The  concentration 
is  therefore  called  the  "optimum."  "  For  each  chemical  there  is 
a  certain  optimum  concentration  in  which  the  Paramecia  are  not 

caused  to  react.  Passage  from 
this  optimum  to  regions  of 
either  greater  or  less  concentra- 
tion causes  the  avoiding  reac- 
tion, so  that  the  animals  tend 
to  remain  in  the  region  of  the 
optimum,  and  if  this  region  is 
small,  to  form  here  a  dense  col- 
lection "  (62,  p.  66,  Fig.  35). 

Paramecia  may  give  any  one 
of   three   reactions    to    contact 
stimuli;  the  first  two  are  nega- 
FIG.  36.  Paramecium  at  rest  with  an-   tive,  the  third  positive,     (i)  If 
terior  end  against  a  mass  of  bac-   Paramecium  swims  against  an 
terial  zoogloea  (a),  showing  the  obstacle,  or  if  the  anterior  end, 
currents  produced  by  the  cilia.    ^^^  ^  sensitive    than 

(From  Jennings.) 

the  other  parts  of  the  body,  is 

touched  with  a  glass  rod,  the  avoiding  reaction  is  given.  (2)  When 
any  other  part  of  the  body  is  stimulated  in  a  like  manner, 
the  animal  may  simply  swim  forward.  (3)  Frequently  a  Para- 
mecium, upon  striking  an  object  when  swimming  slowly,  comes  to 
rest  with  its  cilia  in  contact  with  the  object  (Fig.  36).  This 
positive  reaction  often  brings  the  animals  into  an  environment 
rich  in  food. 

Paramecia  do  not  respond  in  any  way  to  ordinary  visible  light, 
but  give  the  avoiding  reaction  when  ultra-violet  rays  are  thrown 
upon  them;  if  unable  to  escape,  death  ensues  in  from  ten  to  fifty 
seconds  (58). 

The  optimum  temperature  for  Paramecium  lies,  under  ordinary 


PARAMECIUM 


conditions,  between  24°  and  28°  C.  A  number  of  animals  placed 
on  a  slide,  which  is  heated  at  one  end,  will  swim  about  in  all  direc- 
tions, giving  the  avoiding  reaction  where  stimulated,  until  they 
become  oriented  so  as  to  move  toward  the  cooler  end.  This  is  the 
method  of  trial  and  error,  that  is,  the  animal  tries  all  directions 
until  the  one  is  discovered  which  allows  it  to 
escape  from  the  region  of  injurious  stimula- 
tion. 

Gravity  in  some  unknown  way  causes  Parame- 
cia  to  orient  themselves  with  their  anterior  ends 
pointed  upward.  This  brings  them  near  the 
surface  of  the  water.  If  a  number  are  equally 
distributed  in  a  test  tube  of  water,  they  will 
gradually  find  their  way  to  the  top  (Fig.  37). 
The  most  probable  theory  to  account '  for  this 
is  that  the  substances  within  the  body,  being 
of  different  specific  gravities,  move  about  when 
the  animal  changes  its  position  and  act  as  stim- 
uli, relief  being  obtained  only  when  the  animal 
has  placed  its  body  with  its  long  diameter 
perpendicular  to  the  earth's  surface  and  its 
anterior  end  up  (68).  Under  certain  condi- 
tions Paramecium  exhibits  positive  geotropism. 

If  Paramecia  are  placed  in  running  water, 
they  orient  themselves  with  the  anterior  ends 
upstream  and  swim  against  the  current.  This 
is  probably  caused  by  the  interference  of  the 
current  with  the  beating  of  the  cilia,  for  as  soon  as  an  animal 
reaches  a  position  with  anterior  end  upstream,  the  water  no 
longer  tends  to  reverse  the  cilia. 

Paramecia  may  be  subjected  to  an  electric  current  in  the  labora- 
tory, but  they  are  never  influenced  by  it  under  normal  conditions. 
A  weak  current  causes  a  movement  toward  the  cathode;  a  strong 
current  reverses  the  direction  of  the  beating  of  the  cilia  and  causes 
the  animals  to  swim  backward  toward  the  anode.  Many  other 


FIG.  37.  Paramecia 
collected  at 
the  top  of  a 
vertical  tube. 
(From  Jen- 
nings after 
Jensen.) 


O 


78  AN  INTRODUCTION  TO  ZOOLOGY 

interesting  phenomena  might  be  cited,  but  the  entire  subject  is 
too  complex  for  brief  discussion. 

Frequently  Paramecium  may  be  stimulated  in  more  than  one 
way  at  the  same  time.  For  example,  a  specimen  which  is  in 
contact  with  a  solid,  is  acted  upon  by  gravity  and  may  be  acted 
upon  by  chemicals,  heat,  currents  of  water,  and  other  stimuli 
(Fig.  38).  It  has  been  found  that  gravity  always  gives  way  to 

other  stimuli,  and  that  if  more 
than  one  other  factor  is  at  work 
the  one  first  in  the  field  exerts 
the  greater  influence. 

Both  the  spontaneous  activi- 
ties   and   reactions   due  to  ex- 
ternal stimuli  are  due  to  changes 
FIG.  38.  Paramecia  stimulated  by  a    jn  the  internal  condition  of  the 
chemical  and  by  contact  at  the         .       ,       ,-,,       .  7      .  7     .     , 

™/     ,        ,         ,    animal.      Ine  physiological  con- 
same  time.     They  have  formed  r   ^          6 

a  ring  about  a  bubble  of  CO2  dition  of  Paramecium,  therefore, 

and  have  then  come  to  rest  determines  the  character  of  its 

against    the    glass    supporting  response.         This     physiological 

rods,  forming  two  dense  groups.  gtate    js    a    dynamic   condition, 

changing  continually  with   the 

processes  of  metabolism  going  on  within  the  living  substance  of 
the  animal.  Thus  one  physiological  state  resolves  itself  into 
another;  this  "  becomes  easier  and  more  rapid  after  it  has  taken 
place  a  number  of  times  "  (62,  p.  291),  giving  us  grounds  for  the 
belief  that  stimuli  and  reactions  have  a  distinct  effect  upon  suc- 
ceeding responses. 

"  We  may  sum  up  the  external  factors  that  produce  or  deter- 
mine reactions  as  follows:  (i)  The  organism  may  react  to  a 
change,  even  though  neither  beneficial  nor  injurious.  (2)  Any- 
thing that  tends  to  interfere  with  the  normal  current  of  life 
activities  produces  reactions  of  a  certain  sort  ('  negative  ')• 

(3)  Any  change  that  tends  to  restore  or  favor  the  normal  life 
processes  may  produce  reactions  of  a  different  sort  ('  positive  ')• 

(4)  Changes  that  in  themselves  neither  interfere  with  nor  assist 


PARAMECIUM  79 

the  normal  stream  of  life  processes  may  produce  negative  or 
positive  reactions,  according  as  they  are  usually  followed  by 
changes  that  are  injurious  or  beneficial.  (5)  Whether  a  given 
change  shall  produce  reaction  or  not  often  depends  on  the  com- 
pleteness or  incompleteness  of  the  performance  of  the  metabolic 
processes  of  the  organism  under  the  existing  conditions.  This 
makes  the  behavior  fundamentally  regulatory  "  (62,  p.  299). 

Heredity  in  Paramecium.  —  Most  of  the  complex  phenomena 
of  life  are  exhibited  by  the  one-celled  animals,  and  it  is  not  strange 
to  find  that  the  offspring  of  the  Protozoa  resemble  their  parents. 
This  "  resemblance  of  child  to  parent"  (51,  p.  309)  is  called  heredity. 
The  descendants  of  a  living  organism  are  not  exact  copies  of  the 
progenitor,  but  differ  in  various  minor  details;  "  the  difference 
between  child  and  parent  is  called  variation  "(51,  p.  309).  Para- 
mecium has  been  made  the  subject  of  a  thorough  test  with  regard 
to  hereditary  phenomena  by  Professor  Jennings  (65).  This  in- 
vestigator studied  and  measured  over  ten  thousand  Paramecia 
which  were  carefully  bred  in  the  laboratory.  In  a  "  wild  "  lot 
of  Paramecia  eight  distinct  races  were  found.  Each  race  con- 
sisted of  individuals,  which,  though  affected  by  their  environment, 
maintained  a  certain  average  size  which  was  inherited.  Charac- 
teristics that  were  acquired  by  the  Paramecia  were  not  handed 
down  to  their  offspring,  but  were  lost  when  the  animals  were 
reorganized  during  reproduction.  It  is  thus  the  "  fundamental 
constitution  of  the  race,"  and  not  the  various  external  influences 
which  determine  the  characteristics  of  each  new  generation. 


CHAPTER  VI 
OTHER  PROTOZOA 

i.  CLASSIFICATION  OF  THE  PROTOZOA 

"  A  PROTOZOON  is  a  primitive  animal  organism  usually  con- 
sisting of  a  single  cell,  whose  protoplasm  becomes  distributed 
among  many  free  living  cells.  These  reproduce  their  kind  by 
division,  by  budding  or  by  spore  formation,  the  race  thus  formed 
passing  through  different  form  changes  and  the  protoplasm 
through  various  stages  of  vitality  collectively  known  as  the  life 
cycle  "  (78,  p.  17). 

Protozoons  may  be  separated  into  groups  according  to  the 
presence  or  absence  of  locomotor  organs  and  the  character  of 
these  when  present.  Four  divisions  are  usually  recognized : 
(i)  Rhizopoda  with  pseudopodia,  (2)  Mastigophora  with  flagella, 
(3)  Infusoria  with  cilia,  and  (4)  Sporozoa,  which  are  parasitic  in 
cells  and  without  motile  organs. 

The  size  of  Protozoa  ranges  within  wide  limits.  Some  are 
very  large,  for  example,  a  parasite,  Porospora  gigantea,  which 
lives  in  the  alimentary  canal  of  the  lobster,  reaches  two  thirds  of 
an  inch  in  length;  some  are  just  at  the  limit  of  vision  with  the 
most  powerful  microscopes,  whereas  others,  like  the  yellow  fever 
parasite,  doubtless  exist,  though  they  have  never  been  seen. 
Individuals  belonging  to  one  species  vary  in  size  according  to 
their  food  conditions  and  age.  The  variations  in  structure  and 
life  histories  of  the  Protozoa  are  so  numerous  as  to  preclude  de- 
scription in  a  book  of  this  character.  We  shall  therefore  give  an 
abbreviated  classification  of  the  phylum  and  consider  typical 
examples  under  each  class. 

80 


OTHER  PROTOZOA  8 1 


Phylum  Protozoa 

Class  i.   Rhizopoda.     Type:  Ameba  proteus.     Protozoa  usually 
simply  in  structure,  moving  by  means  of  pseudopodia; 
protoplasm  naked. 
Order  Gymnameba.     Without  shells. 

Family  Amebidae.     Pseudopodia  lobose. 

Genus  Ameba.     Ectoplasm  and  endoplasm  distinct. 
Species  proteus.     (Named  after  a  sea  god  who  had  the 
power  of  assuming  different  shapes.) 

Class  2.  Mastigophora.  Types:  Euglenamridis,  Volvox  globator. 
Protozoa  with  one  or  more  flagella;  simplest  forms 
resemble  bacteria. 

Order  Euglenida.  Large  forms  with  one  or  two  flagella; 
mouth  at  base  of  flagellum  through  which  contractile  vacuole 
opens  to  outside. 

Family  Euglenidae.    Elongate;  one  flagellum;  red  eye  spot; 

green  chromatophores. 
Genus  Euglena.     Both  ends  contracted. 

Species  viridis  (green). 

Order  Volvocina.     Colonial  forms  with  two  flagella. 
Family  Volvocaceae. 

Genus  Volvox.     Colonies  of  many  cells  in  a  sphere. 
Species  globator  (a  ball). 

Class  3.   Infusoria.     Type:   Paramecium  caudatum.     Locomotor 
organs,  cilia,  permanent  or  limited  to  young  stages; 
two  kinds  of  nuclei,  macronuclei  and  micronuclei. 
Order  Holotrichida.     Cilia  similar  all  over  body;   trichocysts 
present. 

Family  Paramecidae.     Oral  groove  present. 

Genus  Paramecium.    Mouth  near  middle  of  body;  phar- 
ynx short;  oral  groove  oblique. 
Species  caudatum  (tailed). 


82  AN  INTRODUCTION  TO  ZOOLOGY 

Class  4.  Sporozoa.1    Type:    Plasmodium  vivax.     Parasitic  Pro- 
tozoa; no  motile  organs. 

Order  Hemosporidia.     Blood-dwelling  Sporozoa. 
Genus  Plasmodium. 
Species  vivax  (lively). 

2.    EUGLENA 

(Euglena  viridis  Ehrbg.) 

Euglena  viridis  is  the  animal  usually  selected  in  preparatory 
courses  to  represent  the  Class  Mastigophora.  It  is  found  in 
fresh-water  ponds  and  may  appear  in  cultures  prepared  as  de- 
scribed on  page  38.  It  is  green  in  color,  and,  though  a  single 
animal  cannot  be  seen  with  the  naked  eye,  when  a  great  many  are 
massed  together  they  impart  a  green  tint  to  the  water. 

Anatomy.  —  Euglena  (Fig.  39)  is  a  single  elongated  cell  pointed 
at  the  posterior,  and  blunt  at  the  anterior  end.  Two  kinds  of 
cytoplasm  may  be  distinguished  in  Euglena  as  in  Ameba  and 
Paramecium,  a  dense  outer  layer,  the  ectosarc,  and  a  central  mass, 
the  endosarc,  which  is  more  fluid.  A  thin  cuticle  is  present,  as  in 
Paramecium,  covering  the  entire  surface  of  the  body.  Parallel 
thickenings  of  this  cuticle  run  obliquely  around  the  animal, 
making  it  appear  striated  (B).  A  little  to  one  side  of  the  center 
of  the  anterior  blunt  end  of  the  body  is  a  funnel-shaped  depres- 
sion known  as  the  mouth  (A,  m.).  At  the  bottom  of  this  depres- 
sion is  an  opening  which  leads  into  a  short  duct  called  the  gullet. 

1  Only  one  of  the  protozoan  types  (Plasmodium  vivax)  discussed  in  this 
book  is  parasitic.  There  are,  however,  parasitic  species  in  each  of  the  classes 
of  Protozoa.  For  example,  the  Rhizopod,  Entameba  histolytica,  is  held  re- 
sponsible for  amebic  dysentery  ;  the  Flagellate,  Trypanosoma  gambiense,  is  a 
blood  parasite  which  causes  sleeping  sickness  in  Africa  ;  and  the  Ciliate, 
Balantidium  coli,  which  is  found  in  the  alimentary  canal  of  man,  is  supposed 
to  play  a  part  in  catarrhal  inflamation  of  the  intestine.  Each  class  of  Pro- 
tozoa contains  many  other  parasites  of  both  man  and  the  lower  animals,  some 
being  apparently  harmless,  whereas  others  are  dangerous  and  frequently  fatal. 


EG.  39.  Euglena  viridis.  A,  view  of  free-swimming  specimen  showing  de- 
tails of  structure  ;  B,  another  animal  showing  change  of  shape  and  stria- 
tions  ;  C  and  D,  outlines  showing  stages  of  contraction  ;  E,  reproduction 
by  longitudinal  fission  ;  F  and  G,  division  within  a  cyst:  am.,  pyrenoids 
with  sheaths  of  paramylum ;  chr.,  chromatophors ;  c.v.,  contractile 
vacuoles ;  e.,  stigma;  m.,  mouth;  «.,  nucleus;  r.,  reservoir.  (A-D 
from  Bourne  ;  E-G.  from  Bourne  after  Stein.) 


84  AN  INTRODUCTION  TO  ZOOLOGY 

This  in  turn  enters  a  large  spherical  vesicle,  the  reservoir  (r.~), 
into  which  several  minute  contractile  vacuoles  (A,  c.  v.)  discharge 
their  contents. 

A  conspicuous  structure  in  Euglena  is  the  red  eye  spot  or  stigma 
(Fig.  39,  A,  e.).  This  is  placed  near  the  inner  end  of  the  gullet 
close  to  the  reservoir.  It  consists  of  protoplasm  in  which  are 
embedded  a  number  of  granules  of  hamato  chrome.  The  anterior 
end  of  the  body  of  Euglena  is  said  to  be  more  sensitive  to  light 
than  any  other  part,  and  it  is  supposed  by  some  that  the  stigma 
functions  as  a  rather  primitive  visual  organ.  This  view  is  made 
probable  by  the  presence  of  lens-like  paramylum  grains  just 
anterior  to  it.  The  haematochrome  also  has  many  of  the  charac- 
teristics of  the  pigments  in  the  eyes  of  higher  organisms.  If 
kept  in  the  dark,  Euglena  soon  loses  its  red  pigment.  A  recent 
view  is  that  the  haematochrome  shades  a  sensitive  particle  of 
protoplasm. 

Euglena  contains  a  single  oval  nucleus  (Fig.  39,  n.)  lying  in  a 
definite  position  a  little  posterior  to  the  center  of  the  body.  It 
has  a  distinct  membrane,  and  contains  a  central  body  which  has 
been  called  a  nucleolus  (E),  but  probably  is  not,  since  it  functions 
as  a  division  center  during  mitosis. 

Euglena  derives  its  green  color  from  a  number  of  oval  disks 
suspended  in  the  protoplasm.  These  are  known  as  chromato- 
phores  (Fig.  39  A,  chr.).  They  are  arranged  about  a  collection  of 
granules  situated  in  the  center  of  the  body,  and  contain  chloro- 
phyll, which  is  diffused  throughout  their  protoplasmic  contents. 
They  manufacture  food  by  a  process  common  in  green  plants  but 
rare  in  animals,  called  photosynthesis  (see  p.  18  and  Fig.  3). 
The  chlorophyll  is  able,  in  the  presence  of  light,  to  break  down  the 
carbonic  acid  (CO2) ,  thus  setting  free  the  oxygen,  and  to  unite  the 
carbon  with  water,  forming  a  substance  allied  to  starch  called 
paramylum  (A  and  B,  am.).  If  specimens  are  kept  in  good  light 
continually,  a  large  amount  of  paramylum  will  be  stored  up  for 
future  use,  being  laid  down  around  some  granules  of  proteid  sub- 
stance near  the  center  of  the  body.  These  granules  are  called 


OTHER  PROTOZOA  85 

pyrenoids.  Both  the  pyrenoids  and  chromatophores  are  perma- 
nent cell  structures  and  increase  in  number  by  division  and  not  by 
the  origin  of  new  ones  from  the  other  parts  of  the  body. 

Locomotion.  —  Euglena  changes  its  shape  frequently,  becom- 
ing shorter  and  thicker,  and  shows  certain  squirming  movements. 
These  prove  that  it  possesses  considerable  elasticity,  since  the  nor- 
mal shape  is  regained  if  enough  water  is  present.  Often  in  a 
favorable  specimen,  a  thread-like  structure  may  be  seen  project- 
ing from  the  anterior  end  of  the  body  and  bending  to  and  fro, 
drawing  the  animal  after  it.  This  is  the  flagellum.  It  arises  from 
a  number  of  branching  root-like  fibrils  within  the  body,  passes 
through  the  wall  of  the  mouth  depression,  and  extends  forward  to 
a  distance  often  equal  to  the  length  of  the  animal.  The  part  of  the 
flagellum  outside  of  the  body  is  composed  of  four  contractile 
fibrils  which  are  wound  together  spirally  (Fig.  25).  The  con- 
traction of  these  fibrils  is  supposed  to  produce  all  of  the  move- 
ments characteristic  of  this  structure  (56).  If  the  flagellum 
cannot  be  seen  in  the  living  animal,  a  little  iodine  placed  under 
the  cover  glass  will  help  to  bring  it  out. 

Nutrition.  —  Although  Euglena  has  a  mouth  and  gullet,  it  is 
very  doubtful  if  any  food  is  taken  in.  Food  is  manufactured  as 
in  green  plants,  by  the  aid  of  the  chlorophyll  in  the  chromato- 
phores. This  mode  of  nutrition  is  known  as  holophytic.  That  all 
the  food  necessary  for  the  life  of  the  animal  is  not  procured  in  this 
way  is  shown  by  the  fact  that  the  animal  is  able  to  live  in  the  da'rk 
for  over  a  month,  whereas  chlorophyll  demands  light  before  the 
production  of  paramylum  is  possible.  This  seems  to  indicate 
that  organic  substances  in  solution  are  absorbed  through  the 
surface  of  the  body,  that  is,  saprophytic  nutrition  supplements  the 
holophytic.  The  nutrition  of  Euglena  differs  from  that  of  the 
majority  of  animals,  since  the  latter  live  by  ingesting  solid  par- 
ticles of  food  and  are  said  to  be  holozoic. 

Encystment.  —  Occasionally  Euglena  are  found  which  have 
become  almost  spherical  and  are  surrounded  by  a  rather  thick 

latinous  covering  which  they  have  secreted.     Such  an  animal 


86 


AN  INTRODUCTION  TO  ZOOLOGY 


is  said  to  be  encysted  (Fig.  39  F  and  G) .  In  this  condition  periods 
of  drought  are  successfully  passed,  the  animals  becoming  active 
when  water  is  again  encountered.  Usually  in  cultures  brought 
into  the  laboratory  many  cysts  are  found  on  the  sides  of  the  dish. 


FIG.  40.  Diagram  of  the  reaction  of  Euglena  when  the  light  is  decreased. 
The  organism  is  swimming  forward  at  i  ;  when  it  reaches  2  it  is  shaded. 
It  thereupon  swerves  toward  the  dorsal  side,  at  the  same  time  continu- 
ing to  revolve  on  the  long  axis,  so  that  its  anterior  end  describes  a  circle, 
the  Euglena  occupying  successively  the  positions  2-6.  From  any  of 
these  it  may  start  forward  in  the  directions  indicated  by  the  arrows. 
(From  Jennings.) 

Encystment  frequently  takes  place  without  any  apparent  cause, 
the  animal  resting  in  this  condition  for  a  time  and  then  emerg- 
ing again  to  its  free  swimming  habit.  Before  encystment  the 
flagellum  is  thrown  off,  a  new  one  being  produced  when  activity 
is  again  resumed. 


OTHER  PROTOZOA  87 

Reproduction.  —  Reproduction  in  Euglena  takes  place  by 
binary  longitudinal  division  (Fig.  39  E).  The  nucleus  divides 
by  a  primitive  sort  of  mitosis.  The  body  begins  to  divide  at  the 
anterior  end.  The  old  flagellum  is  retained  by  one  half,  while  a 
new  flagellum  is  developed  by  the  other.  Often  division  takes 
place  while  the  animals  are  in  the  encysted  condition.  One  cyst 
usually  produces  two  Euglena  although  these  may  divide  while 
still  within  the  old  cyst  wall,  making  four  in  all,  while  recent 
observers  have  recorded  as 
many  as  thirty-two  young 
flagellated  Euglena  which 
escaped  from  a  single  cyst. 

Behavior. — Euglena  swims 
through  the  water  in  a  spiral 
path.  The  effect  of  this 
course  is,  as  we  found  in 

Paramecium  (p.  64),  the  pro-  r  -p.. 

c          TIG.  41.  Diagram  showing  the  reaction 

duction  of  a  perfectly  straight  of  EugletUB  to  light.  The  light  comes 
course  through  the  trackless  from  the  direction  indicated  by  the 
water.  When  stimulated  by  arrows,  while  the  opposite  side  of 

a  change  in  the  intensity  of  the  vessel  is  shaded>  as  indicated 

,,      r  T,     ^     7          .     ,,  by  the  dots.     The  Euglence  gather 

the  light,  Euglena,  in  the  ma-  •  *•.•*        j-  * 

0  in  the  intermediate  region  across  the 

jonty  of  cases,  stops  or  moves          middle.     (From  Jennings.) 
backward,  turns  strongly  to- 
ward the  dorsal  side,  but  continues  to  revolve  on  its  long  axis. 
The  posterior  end  then  acts  as  a  pivot  while  the  anterior  end 
traces  a  circle  of  wide  diameter  in  the  water.     The  animal  may 
swim  forward  in  a  new  direction  from  any  point  in  this  circle. 
This  is  the  avoiding  reaction  (Fig.  40). 

Euglena  is  very  sensitive  to  light  and  is  a  favorable  object  for 
the  study  of  phototropism.  It  swims  toward  an  ordinary  light 
such  as  that  from  a  window,  and  if  a  culture  containing  Euglena  is 
examined,  most  of  the  animals  will  be  found  on  the  side  toward 
the  brightest  light.  This  is  of  distinct  advantage  to  the  animal, 
since  light  is  necessary  for  the  assimilation  of  carbon  dioxide 


88  AN  INTRODUCTION  TO  ZOOLOGY 

by  means  of  its  chlorophyll.  Euglena  will  swim  away  from  the 
direct  rays  of  the  sun.  Direct  sunlight  will  kill  the  organism  if 
allowed  to  act  for  a  long  time.  If  a  drop  of  water  containing 
Euglena  is  placed  in  the  direct  sunlight  and  then  one  half  of  it  is 
shaded,  the  animals  will  avoid  the  shady  part  and  also  the  direct 
sunlight,  both  of  which  are  injurious  to  them,  and  will  remain  in  a 
small  band  between  the  two  in  the  light  best  suited  for  them, 
that  is,  their  optimum  (Fig.  41).  By  shading  various  portions  of 
the  body  of  a  Euglena  it  has  been  found  that  the  region  in  front 
of  the  eye  spot  is  more  sensitive  than  any  other  part.  It  should 
be  noted  that  when  Euglena  is  swimming  through  the  water  it  is 
this  anterior  end  which  first  reaches  an  injurious  environment; 
the  animals  give  the  avoiding  reaction  at  once,  and  are  thus  carried 
out  of  danger. 

3.  PLASMODIUM 

(Plasmodium  vivax  Grassi) 

The  Sporozoa  are  parasitic  Protozoa  and  are  responsible  for 
many  of  the  most  malignant  animal  diseases.  One  of  the  best 
known  of  all  Sporozoons  is  Plasmodium  vivax,  which  causes  Mala- 
rial Fever.  This  minute  animal  was  discovered  in  the  blood  of 
malaria  patients  by  a  French  military  doctor,  Laveran.  It  was 
suggested  by  this  investigator,  in  1891,  that  the  parasite  is  prob- 
ably transmitted  from  man  to  man  by  some  blood-sucking  insects, 
and  this  hypothesis  was  proved  to  be  correct  by  the  work  of  Major 
Ross  in  1899.  Not  only  was  it  demonstrated  that  malaria  is 
spread  by  insects,  but  it  was  proved  that  human  beings  can  only 
become  infected  by  the  bite  of  a  mosquito  belonging  to  the  genus 
Anopheles.  The  two  most  common  genera  of  mosquitoes  are 
Culex  and  Anopheles.  One  of  the  easiest  methods  of  distinguish- 
ing one  from  the  other  is  by  observing  their  position  when  at 
rest.  It  will  be  found  that  the  harmless  Culex  holds  its  abdomen 
approximately  parallel  with  the  surface  on  which  it  alights, 
whereas  the  abdomen  of  Anopheles  is  held  at  an  angle. 

There  are  three  well-known  types  of  malaria;   these  may  be 


VI. 


FIG.  42.  Diagram  illustrating  the  life  history  of  the  Malarial  Fever  parasite.  The  stages 
shown  above  the  line  of  dashes  are  passed  through  in  the  blood  of  a  human  being  ;  those 
below  the  line,  in  the  body  of  an  Anopheles,  mosquito.  For  explanations  of  the  stages, 
see  pages  89-91.  (From  Minchin  in  Lankester's  Treatise  after  various  authors.) 


OTHEK  PROTOZOA  89 

recognized  by  the  intervals  between  successive  chills,  (i)  Ter- 
tian fever,  caused  by  Plasmodium  vivax,  is  characterized  by  an 
attack  every  forty-eight  hours;  (2)  quartan  fever,  caused  by 
Plasmodium  malarice,  with  an  attack  every  seventy-two  hours, 
and  (3)  estiw-autumnal  or  pernicious  fever ,  caused  by  Plasmodium 
falciparum,  produces  attacks  daily  or  more  or  less  constant  fever. 
The  life  histories  of  these  three  species  of  Plasmodium  differ 
very  slightly  one  from  another.  Plasmodium  vivax  has  been 
selected  for  presentation  here  as  a  type  of  the  group  Sporozoa. 
The  various  stages  that  occur  during  the  life  cycle  of  this  parasite 
offer  a  mass  of  detail  that  can  only  be  adequately  discussed  in  a 
larger  work,  so  we  shall  simply  give  a  brief  outline  of  the  chief 
phases. 

Tertian  fever  is  transmitted  by  diseased  female  mosquitoes 
only.  The  mouth  parts  of  these  insects  are  adapted  for  piercing. 
When  they  have  been  thrust  into  the  skin  of  the  victim,  a  little 
saliva  is  forced  into  the  wound.  This  saliva  contains  a  weak 
poison  which  is  supposed  to  prevent  the  coagulation  of  the  blood 
and  thus  the  clogging  of  the  puncture.  Blood  is  sucked  up  by 
the  mouth  parts  into  the  alimentary  canal  of  the  mosquito; 
this  process  occupies  from  two  to  three  and  a  half  minutes.  With 
the  saliva  a  number  of  parasites,  which  were  stored  in  the  salivary 
glands  of  the  insect,  find  their  way  into  the  wound.  Constant 
reference  to  Figure  42  will  make  the  following  description 
clear. 

At  the  time  of  their  entrance  the  parasites  are  slender  boat- 
shaped  cells  pointed  at  both  ends,  and  are  called  sporozoites 
(XIX).  These  sporozoites  immediately  penetrate  the  red  blood 
corpuscles  by  means  of  wriggling  movements.  Inside  of  the 
blood  corpuscle  the  sporozoite  becomes  ameboid  in  shape,  and 
begins  to  feed  on  the  surrounding  protoplasm  (I).  As  it  grows 
larger  a  vacuole  develops,  giving  it  a  ringlike  appearance  (II  and 
III).  When  fully  grown  the  parasite  almost  completely  fills  the 
blood  corpuscle  and  its  ameboid  movements  cease  (IV,  V,  and  6). 
Within  its  body  can  be  seen  a  number  of  dark  br,own  granules 


90  AN  INTRODUCTION  TO  ZOOLOGY 

called  melanin  (9  and  10,  p.) ;  these  are  waste  matters  that  have 
accumulated  during  the  growth  period,  being  modified  products 
of  the  digestion  of  haemoglobin. 

Reproduction  now  takes  place  by  a  p  ocess  termed  schizogony 
(I-V  and  6-10),  the  parasite  after  it  enters  the  blood  corpuscle, 
being  known  as  a  schizont.  During  this  process  the  nucleus 
divides  a  number  of  times  without  the  intervention  of  cell  walls 
until  from  twelve  to  sixteen  daughter  nuclei  are  present  (8). 
Each  nucleus,  with  part  of  the  surrounding  cytoplasm,  is  then  cut 
off  from  its  fellows,  resulting  in  the  production  of  a  number  of 
small  cells,  called  merozoites,  inclosed  in  the  membrane  of  the 
corpuscles  (9).  The  melanin  granules  are  not  included  in  the 
daughter  cells,  but  remain  in  the  center  of  the  mother  para- 
site (9,  />.).  The  merozoites  finally  break  through  the  walls  of 
the  corpuscle  and  escape  into  the  blood  plasma.  The  melanin 
granules  are  also  liberated  at  this  time  (10,  />.);  they  are  car- 
ried by  the  blood  to  the  liver,  kidney,  spleen,  lungs,  or  brain,  fre- 
quently resulting  in  the  pigmentation  and  hypertrophy  of  these 
organs.  An  attack  of  fever  coincides  with  the  liberation  of  the 
merozoites,  and  the  interval  between  the  penetration  of  a  blood 
corpuscle  and  the  production  of  merozoites  is  termed  the  period 
of  incubation.  This  period  in  Plasmodium  vivax,  as  stated  above, 
is  forty-eight  hours.  All  the  merozoites  simultaneously  attack 
other  corpuscles  (10  to  I),  producing  the  chill  which  is  a  symptom 
of  malaria. 

Several  asexual  cycles  like  that  just  described  are  passed 
through;  but  finally,  for  some  unknown  reason,  the  merozoites 
develop  into  sexual  phases  (VI-XII).  Part  of  them,  named 
macrogametocytes  (VII,  b.)  become  capable  of  producing  a  single 
female  gamete  or  macrogamete  (X,  b.),  while  the  rest  become 
microgametocytes  (VII,  a.) ,  each  capable  of  producing  as  many  as 
eight  male  elements  or  micro  gametes  (X,  a.).  The  gametocytes 
cannot  produce  gametes  within  the  human  body  but  must  first 
be  sucked  up  into  the  alimentary  canal  of  an  Anopheles  mosquito. 
Here  all  stages  of  the  parasite  except  the  gametocytes  are  di- 


OTHER  PROTOZOA  91 

gested ;  the  latter  seem  stimulated  by  the  digestive  juices  to  fur- 
ther development. 

The  macro gametocyte  now  goes  through  the  process  of  matura- 
tion (seep.  105),  one  small  polar  body  being  extruded  (X,  &.); 
what  remains  is  a  large  macrogamete.  The  nucleus  of  the  micro- 
gametocyte  (IX,  a.  n.)  divides  into  eight  daughter  nuclei  which 
migrate  to  the  periphery  and  enter  the  long  protoplasmic  processes 
that  have,  in  the  meantime,  been  thrust  out  from  the  cell  (X,  a.). 
These  flagelliform  nucleated  processes  then  break  away  from  the 
microgametocyte  as  male  cells,  microgametes  or  spermatozoa, 
and  move  about  until  they  encounter  a  macrogamete  with  which 
they  fuse  (XI).  What  remains  of  the  microgametocyte  slowly 
disintegrates.  Only  one  microgamete  fuses  with  a  macrogamete 
in  the  process  of  fertilization. 

The  result  of  this  fusion  is  a  zygote  which  has  been  given  the 
name  ookinet  (XII) .  This  zygote  becomes  spindle-shaped  (XIII) 
and  makes  its  way  by  vermiform  movements  to  the  epithelial  cells 
lining  the  alimentary  canal  of  the  mosquito;  it  penetrates  these 
and  reaches  the  underlying  tissues.  Here  it  grows  rapidly; 
the  nucleus  divides,  forming  many  daughter  nuclei,  and  by  the 
sixth  day  as  many  cells  are  produced  as  there  are  nuclei  (XIV- 
XVI).  Each  cell  is  a  sporoblast  (XVI,  Sp.  bl.).  The  sporoblast 
forms  by  division  a  number  of  germs,  the  sporozoites  (XVII- 
XVIII) ;  these  mature  in  about  fourteen  days  and  are  then  set 
free  into  the  body  cavity  of  the  mosquito  (XVIII-XIX).  From 
here  they  are  carried  by  the  circulating  plasma  to  all  parts  of  the 
body,  finally  collecting  in  the  anterior  region.  They  make  their 
way  into  the  salivary  glands,  and  are  then  ready  to  pass  into  the 
blood  of  the  human  being  at  the  time  the  mosquito  makes  its 
next  meal. 

Malarial  fever  is  not  present  wherever  Anopheles  is  found;  for 
example,  Anopheles  is  common  in  England,  but  no  malaria  occurs 
there.  It  is  supposed  that  mosquitoes  may  become  immune  to 
all  kinds  of  blood  parasites.  A  mosquito  that  is  able  to  digest 
the  parasite  becomes  harmless  to  man.  The  mosquitoes  of  the 


AN  INTRODUCTION  TO  ZOOLOGY 


genus  Culex  are  able  to  digest  all  stages  of  Plasmodium  vivax,  and 
are  therefore  immune.  On  the  other  hand,  this  mosquito  does 
not  digest  all  of  the  organisms  of  the  malaria  which  attacks  birds 
and  thus  becomes  infected  and  is  the  means  of  transmitting  the 

fever  from  one  bird  to 
another. 

Quinine  is  the  rem- 
edy commonly  used 
against  the  malarial 

a  ^£^    ,_  _  parasite.     It  acts  di- 

rec tly  upon  the 
younger  stages  of  the 
organism,  causing 
death. 


4.   VOLVOX  GLOBATOR 

AND  ITS  ALLIES 


Certain  organisms 
which  lie  on  the 
borderland  between 
plants  and  animals  are 
peculiarly  favorable 

for  illustrating  how  a 

FIG.  43.  Chlamydomonas,     A,  single  vegetative  .  .     ,,   ,               .       , 

cell:  a,  stigma ;  chr.,  chromatophor ;  g.,  fla-  multicellular      animal 

gellum;  k.,  nucleus;  />y., pyrenoid ;  v.,  vac-  llke    Volwx   (Fig.   46) 

uoles.     B,  Four  daughter  cells  still  within  may      have      evolved 

the  cell  wall  of  the  parent;    C,  the  union  frOm     a     single-celled 

of  two  gametes;   D,  zygote.     (From  Olt-  ancestort      These    or- 

manns.) 

ganisms  belong  to  the 

Family  Volvocaceae.  They  are  important  not  only  for  their  phy- 
logenetic  significance  but  also  because  they  illustrate  the  evolu- 
tion of  sex.  Starting  with  the  unicellular  Chlamydomonas,  the 
line  of  evolution  passes  through  Spondylomorum,  Pandorina, 
Eudorina,  and  finally  ends  with  Volwx  (76). 


OTHER  PROTOZOA 


Chlamydomonas  (Fig.  43)  is  found  in  ponds,  ditches,  and  rain 
pools.  It  is  oval  in  shape,  green  in  color,  and  swims  about  by 
means  of  two  flagella  (g)  which  project  from  one  end  (Fig.  43  A). 
The  central  mass  of  protoplasm  is  covered  by  a  definite  wall  of 
cellulose.  Within  the  cells  are  a  nucleus  (k),  two  small  contractile 
vacuoles  (v),  a  large  pyrenoid  (py),  a  red  pigment  spot  (a),  the 
stigma,  and  chromatophors  (chr). 

Reproduction  takes  place  in  two  ways.  First,  a  cell  comes  to 
rest  and  its  contents  divide  into  two,  four  or  eight  daughter  cells 
which  assume  the  characteristics  of 
the  parent  organism  and  lead  a 
separate  existence  (Fig.  43  B).  Sec- 
ond, a  cell  may  divide  into  from  six- 
teen to  sixty-four  cells  without  walls; 
these  are  smaller  than  the  vegetative 
cells,  but  resemble  one  another. 
They  are  known  as  gametes.  The 
gametes  fuse  in  pairs  (Fig.  43  C), 
forming  spherical  bodies  with  heavy 
walls  called  zygotes  (D);  after  a 
period  of  rest,  the  contents  of  the 
zygotes  divide  into  a  number  of 
parts  which  escape  from  the  walls 
and  grow  into  vegetative  cells. 

Spondylomorum  (Fig.  44)  differs  from  Chlamydomonas  in  a 
number  of  particulars,  the  most  noticeable  being  its  colonial 
habit.  Specimens  may  be  found  in  situations  similar  to  those 
cited  for  Chlamydomonas.  When  brought  into  the  laboratory 
they  collect  at  the  side  of  the  culture  dish  nearest  the  window, 
reacting  positively  to  light  of  moderate  intensity.  Sixteen  cells 
form  a  colony;  they  are  arranged  in  four  rows  of  four  each,  alter- 
nating, as  shown  in  Figure  44.  Each  cell  appears  to  be  exactly 
like  every  other  cell ;  it  possesses  four  flagella  and  has  a  chloro- 
phyll body  which  enables  it  to  manufacture  its  own  food.  All  of 
the  somatic  functions  are  carried  on  by  each  cell  independently 


FIG.  44.  Spondylomorum. 
(From  Oltmanns.) 


94  AN  INTRODUCTION  TO  ZOOLOGY 

of  all  the  others.  Reproduction  likewise  takes  place  in  each 
cell. 

So  far  as  known  reproduction  of  only  one  kind  occurs;  this  is 
the  simultaneous  longitudinal  division  of  each  cell  into  two,  four, 
eight,  and  finally  sixteen  daughter  cells.  These  daughter  cells 
do  not  separate  as  do  the  daughter  cells  when  Chlamydomonas 
divides,  but  remain  fastened  together  by  a  gelatinous  matrix. 
Each  cell  of  the  mother  colony  thus  produces  a  daughter  colony 
of  sixteen  cells.  If  another  kind  of  reproduction  does  take  place, 
it  is  probably  by  the  union  of  similar  gametes  forming  a  zygote 
similar  to  that  of  Chlamydomonas. 

Pandorina  morum  (Fig.  45)  is  likewise  a  colonial  form  found  in 
fresh-water  ponds.  It  consists  of  sixteen  cells  which  are  held 
together  by  a  gelatinous  matrix,  part  of  which  is  secreted  by  each 
cell  (I).  Each  cell  possesses  one  pair  of  flagella  and  an  eye  spot, 
and  contains  chlorophyll.  It  is  thus  enabled  to  carry  on  all  of  the 
processes  necessary  to  sustain  life,  to  grow,  and  to  reproduce. 

Reproduction  in  this  species  takes  place  in  two  ways.  First, 
as  in  Spondylomorum,  each  cell  may  divide  to  form  two,  four, 
eight,  and  then  sixteen  daughter  cells,  which  become  a  new  colony 
(Fig.  45,  II).  The  new  colonies  escape  from  the  mother  colony 
by  the  dissolution  of  the  gelatinous  envelope,  and  swim  away  to 
lead  a  separate  existence.  This  method  of  reproduction  occurs 
repeatedly,  but  finally  conjugation  is  inaugurated.  Each  of  the 
sixteen  cells  of  the  mother  colony  produces  by  division  sixteen  or 
thirty-two  daughter  cells.  Each  daughter  cell  develops  flagella 
and  separates  not  only  from  the  mother  colony,  but  from  its  sister 
cells  in  the  daughter  colony  (III).  This  separation  results  from 
a  solution  of  not  only  the  gelatinous  envelope  of  the  mother  colony 
but  also  that  which  holds  the  daughter  cells  together.  These 
isolated  cells  are  known  as  gametes.  The  gametes  are  not  all  of 
the  same  size,  some  being  larger  t  han  others.  When  two  gametes 
meet,  they  become  fastened  together  by  their  anterior  ends  and 
gradually  fuse  (IV-V).  The  fused  cells  constitute  a  zygote,  con- 
taining two  eye  spots  and  bearing  four  flagella  (V).  The  flagella 


OTHER  PROTOZOA 


95 


are  soon  cast  off  and  the  zygote  sinks  to  the  bottom  of  the  pond 
where  it  secretes  a  thick  red  wall  about  itself  (VI- VII).  It  re- 
mains in  this  condition  throughout  the  summer,  becoming  dry 
when  the  pool  of  water  in  which  it  lives  evaporates.  In  the 
autumn  activity  is  again  resumed.  The  zygote  produces  from 


FIG.  45.  Pandorina  morum.  I,  vegetative  colony ;  II,  formation  of  daughter 
colonies;  III,  escape  of  gametes;  IV,  V,  VI,  VII,  fusion  of  two 
gametes  to  form  a  zygote ;  VIII,  IX,  production  of  swarm  spores  by  a 
zygote ;  X,  vegetative  colony  formed  by  a  swarm  spore.  (From  Olt- 
manns.) 


96  AN  INTRODUCTION  TO  ZOOLOGY 

one  to  three  swarm  spores  which  escape  from  the  cyst  and  swim 
away  (VIII-IX).  Each  swarm  spore  develops  at  once  by  binary 
division  a  new  colony  of  sixteen  cells  (X). 

The  gametes  of  Pandorina  are  not  equal  in  size  as  in  Chlamy- 
domonas,  but  appear  to  be  of  two  kinds,  some  larger  than  others. 
Where  such  a  condition  exists  the  smaller  gametes  are  called 
male  cells  or  sperms,  the  larger,  female  cells  or  eggs.  Sexual 
reproduction  consists  in  the  union  of  a  male  cell  with  a  female 
cell,  and  certain  investigators  believe  the  larger  gametes  of 
Pandorina  tend  to  fuse  with  the  smaller.  If  this  is  true,  we  have 
in  this  organism  the  beginning  of  the  evolution  of  sexual  repro- 
duction. 

In  Eudorina  elegans,  another  member  of  the  family  Volvo- 
caceae,  this  distinction  between  male  and  female  gametes  is  more 
clearly  seen.  Eudorina  is  a  spherical  colony  containing  thirty- 
two,-  rarely  sixteen  cells  which  resemble  the  cells  of  Pandorina. 
Reproduction  by  simple  division  takes  place  as  in  Pandorina. 
This  cannot  go  on  indefinitely,  for  finally  some  colonies  are 
found  whose  cells  have  grown  larger  than  usual.  These  are 
female  colonies,  and  the  cells  are  macro  gametes.  Other  colonies 
produce  microgametes;  each  of  the  thirty-two  cells  of  these 
male  colonies  divides,  producing  a  flat  plate  of  sixteen  or  thirty- 
two  spindle-shaped  microgametes  each  with  a  pair  of  flagella 
extending  from  its  anterior  end.  When  a  plate  of  microgametes 
encounters  a  colony  of  macrogametes  it  becomes  attached  to 
it,  the  microgametes  separate  from  the  plate  and  make  their 
way  through  the  gelatinous  envelopes  of  the  macrogametes, 
thus  fertilizing  them  and  forming  zygotes. 

It  should  be  noted  that  the  organization  of  Eudorina  is  more 
complex  than  that  of  the  other  three  species  considered,  and  that 
this  advance  in  complexity  is  gradual.  In  Eudorina,  also,  the 
small  microgametes  are  perfectly  distinct  from  the  large  macro- 
gametes,  and  fertilization  always  consists  in  the  union  of  a  macro- 
with  a  microgamete.  This  size  difference  is  not  so  evident  in 
Pandorina. 


riG.  46.  Vohox  globator.    A,  a  colony  reproducing  by  means  of  parthenogonidia ;  1-7,  various 
stages  in  the  division  of  a  germ  cell  to  form  a  daughter  colony.     (From  Lang.) 


nt" 


FIG.  46.  Vohox  globator,  B,  a  colony  reproducing  by  means  of  eggs  and 
sperms:  0.,  eggs;  s.,  sperms.  (From  Lang.)  C,  a  single  cell  showing 
protoplasmic  connections,  pi.,  cell  protoplasm;  g.,  jelly;  m.,  lines 
showing  limits  of  cell  envelope.  (From  Oltmanns  after  various  authors.) 


OTHER  PROTOZOA  97 

Finally  in  Volvox  globator  (Fig.  46),  the  specialization  of  the 
body  cells  as  well  as  the  reproductive  cells  reaches  a  stage  in 
which  certain  cells  are  set  apart  for  reproductive  purposes  while 
others  carry  on  the  vegetative  functions  of  the  colony. 

Vofoox  is  a  colonial  organism  found  in  fresh-water  ponds.  It 
can  easily  be  seen  with  the  naked  eye,  being  from  .2  to  .7  mm.  in 
diameter.  It  may  be  compared  with  a  hollow  rubber  ball,  since 
it  is  a  hollow  sphere  consisting  of  a  single  peripheral  layer  of  cells 
embedded  in  a  gelatinous  matrix.  Each  of  the  twelve  thousand 
cells  which  may  be  present  has  a  thick  envelope  separating  it 
from  the  surrounding  cells;  mutual  pressure  gives  these  a  hexa- 
gonal shape  (Fig.  46  A).  Strands  of  protoplasm  connect  each 
cell  with  the  six  cells  that  surround  it  (Fig.  46  C) ;  physiological 
continuity  is  thus  established  between  the  cells,  a  condition  not 
found  in  the  colonies  previously  described.  Most  of  the  cells 
contain  an  eye  spot,  chlorophyll,  a  contractile  vacuole,  and  two 
flagella.  These  are  called  "body"  or  somatic  cells.  They  serve 
to  propel  the  colony  through  the  water  and  to  carry  on  nutritive 
processes,  but  are  incapable  of  reproducing  new  colonies,  although 
in  young  colonies  they  divide,  giving  rise  to  daughter  cells  like 
themselves.  The  production  of  daughter  colonies  is  accomplished 
by  special  reproductive  cells  which  are  set  aside  for  this  purpose. 

Reproductive  cells  give  rise  to  new  colonies  in  two  ways:  (i) 
asexually,  and  (2)  sexually.  The  asexual  method  is  as  follows. 
Certain  cells  of  the  colony  are  larger  than  others  and  lack  flagella 
(Fig.  46  A,  i) ;  a  number  of  these  in  one  colony  increase  in  size, 
and  divide  by  simple  fission  into  a  great  number  of  cells,  produc- 
ing new  colonies  without  being  fertilized  (A,  1-7).  The  cells 
that  act  in  this  way  are  named  parthenogonidia.  The  colonies 
derived  from  them  drop  into  the  central  cavity  of  the  mother 
colony,  where  they  swim  about  for  a  short  time,  finally  escaping 
through  a  chance  opening  in  the  wall. 

After  colonies  have  been  produced  in  this  manner  for  several 
generations  the  sexual  method  of  reproduction  may  be  observed 
(Fig.  46  B).  Colonies  are  found  which  contain  as  many  as  fifty 


98  AN  INTRODUCTION  TO  ZOOLOGY 

of  the  larger  cells  without  flagella.  Some  of  these  grow  larger  and 
may  be  recognized  as  female  cells  or  macrogametes  (o);  others 
produce  by  simple  division  a  flat  plat,  containing  about  one  hun- 
dred and  twenty-eight  spindle-shaped  male  cells  or  microgametes 
(s).  These  escape  into  the  central  cavity  and  fuse  with  the 
macrogametes.  The  zygote  thus  formed  secretes  a  surrounding 
wall  consisting  of  two  layers,  the  outer  of  which  is  reddish  in  color 
and  covered  with  short  spines.  In  this  condition  the  winter  is 
passed.  The  following  spring  the  zygote  breaks  out  of  the  wall 
and  by  division  produces  a  new  colony.  The  smaller  somatic 
cells  contained  in  the  mother  colony  fall  to  the  bottom  and  disin- 
tegrate as  soon  as  the  new  colonies  produced  by  the  fertilized 
germ  cells  have  escaped. 

In  Volvox,  true  somatic  cells  are  encountered  for  the  first  time, 
that  is,  cells  which  function  only  vegetatively  and  are  unable  to 
reproduce  the  colony.  In  the  other  forms  described  every  cell 
has  the  capacity  of  reproducing  the  whole.  Volvox  also  contains 
true  germ  cells,  that  is,  cells  that  have  given  up  nutritive  functions 
to  carry  on  reproduction.  Furthermore,  a  clear  case  of  natural 
death  occurs  in  the  somatic  cells  when  they  fall  to  the  bottom  of 
the  pond  and  disintegrate.  The  bodies  of  higher  animals  consist 
of  many  cells  which  may  be  separated  into  somatic  and  germ  cells. 
The  latter  are  either  male  or  female.  In  most  cases  a  fusion  of  a 
male  cell  with  a  female  cell  is  necessary  before  a  new  animal  can 
be  reproduced.  At  any  rate,  some  of  these  germ  cells  maintain 
the  continuation  of  the  species  by  producing  new  individuals 
while  the  somatic  cells  perish  when  the  animal  dies. 

The  Germ  Plasm  Theory.  —  Figure  47  illustrates  the  theory 
of  the  continuity  of  the  germ-plasm  which  is  held  by  most  zoolo- 
gists at  the  present  time.  By  germ-plasm  is  meant  that  part  of 
the  protoplasm  which  is  set  aside  for  reproductive  purposes  and 
determines  that  the  offspring  shall  resemble  their  parents; 
this  special  material  is  stored  in  the  germ  cells.  In  the  spring 
of  the  year  the  Volvox  race  is  represented  by  fertilized  eggs 
(zygotes)  only;  each  of  these  divides,  producing  an  animal  con- 


OTHER  PROTOZOA  99 

taining  as  many  as  twelve  thousand  cells,  a  few  of  which  are 
reproductive  cells  and  contain  the  germ-plasm.  In  the  autumn 
the  germ-plasm  is  segregated  in  the  eggs  and  spermatozoa,  which 
fuse  two  by  two,  an  egg  with  a  sperm,  producing  zygotes.  The 
somatic  or  body  cells  fall  to  the  bottom  and  die,  but  the  zygotes 
live  through  the  winter  and  germinate  in  the  spring,  thus  assuring 
the  continued  existence  of  the  race. 


FIG.  47.  Diagram  to  illustrate  the  theory  of  the  continuity  of  the  germ-plasm 
by  means  of  Volvox.  The  fertilized  egg  or  zygote  (zi)  produces  by  divi- 
sion in  the  spring  a  colony  (c)  containing  a  great  many  cells,  some  of 
which  are  set  aside  for  reproductive  purposes.  These  produce  new  colo- 
nies (n.c.)  during  the  summer  by  the  asexual  method.  Male  cells  (sper- 
matozoa, sp)  and  female  cells  (eggs,  e)  are  also  formed.  In  the  autumn 
the  body  dies,  but  the  fertilized  germ  cells  (zz)  produced  by  the  union  of 
eggs  and  spermatozoa  (/)  survive  the  winter,  dividing  to  form  new  colo- 
nies the  next  spring. 

Certain  processes  in  the  development  of  the  fertilized  eggs  of 
some  of  the  more  complex  animals  strengthen  the  belief  in  the 
passage  of  the  germ-plasm  from  one  generation  to  another,  and 
seem  to  indicate  that  the  body  is  not  the  producer  of  the  germ 
cells,  but  is  simply  a  vehicle  which  carries  them  about  until  they 
reach  maturity.  The  body  then  dies  and  disintegrates,  the 
important  function  of  reproduction,  and  consequently  the  sur- 
vival of  the  race,  being  left  in  charge  of  the  mature  germ  cells. 


CHAPTER  VII 
AN   INTRODUCTION   TO   THE   METAZOA 

i.  CELLULAR  DIFFERENTIATION  —  TISSUES 

THERE  is  no  sharp  line  between  the  Metazoa  and  Protozoa. 
We  have  seen  (Chap.  VI)  that  the  Volvocaceae,  which  are  often 
included  in  the  Phylum  Protozoa,  are  many-celled  colonies,  the 
cells  of  which  are  either  entirely  independent,  as  in  Spondy- 
lomorum  (Fig.  44),  or  are  connected  by  protoplasmic  strands,  as 
in  Volwx  (Fig.  46).  Here  the  cells  of  the  colony  have  ceased  to 
be  independent,  but  have  united  to  form  a  distinct  body  which 
carries  on  nutritive  processes,  produces  germ  cells,  and  then  dies. 
The  cells  of  this  body  are  called  somatic  cells.  Their  functions 
are  locomotor  and  nutritive,  but  not  reproductive.  Such  an 
aggregation  of  cells  is  known  as  a  tissue.  A  tissue  is  an  associa- 
tion of  similar  cells  originating  from  a  particular  part  of  the 
embryo  and  with  special  functions  to  perform.  In  Volwx  there 
is  only  one  kind  of  tissue ;  in  some  of  the  simple  Metazoa  there 
are  two  kinds  of  tissue  resulting  from  the  differentiation  of  the 
somatic  cells;  in  the  majority,  however,  there  are  at  least  three 
and  usually  more.  The  many  different  kinds  of  tissue  in  the 
Metazoa  may  be  classified  according  to  their  structure  and 
functions  into  four  groups. 

(i)  Epithelial  tissue  (Fig.  48  A,  B)  consists  of  cells  which  cover 
all  the  surfaces  of  the  body  both  without  and  within.  In  the 
simpler  animals  this  is  the  only  kind  of  tissue  present.  The 
somatic  cells  of  Volwx  may  be  considered  epithelium.  In  the 
more  complex  animals  epithelial  cells  become  variously  modified 
because  they  are  the  means  of  communication  between  the  or- 
ganism and  its  environment;  nutritive  material  passes  through 


— 6 


FIG.  48.  Examples  of  various  kinds  of  tissues ;  A,  ciliated  epithelium : 
B,  stratified  epithelium  ;  C,  hyaline  cartilage  ;  D,  striated  muscle  fiber , 
E,  nonstriated  muscle  fiber.  (From  Parker  and  Haswell.) 


AN  INTRODUCTION  TO  THE  METAZOA  iol 

them  into  the  body,  and  excretory  products  pass  through  them  on 
their  way  out  of  the  body;  they  also  contain  the  end  organs  of 
the  sensory  apparatus  and  protect  the  body  from  physical  contact 
with  the  outside  world.  In  man  the  cuticle  and  lining  of  the 
alimentary  canal  are  examples  of  epithelium. 

(2)  Supporting  and  Connective  Tissues  (Fig.  48  C)  may  be  en- 
countered in  almost  any  part  of  the  body.    Their  chief  functions 
are  two  in  number;   (i)  they  bind  together  various  parts  of  the 
body,  and  (2)  they  form  rigid  structures  capable  of  resisting 
shocks  and  pressures  of  all  kinds.     These  tissues  consist  largely 
of  non-living  substances,  fibers,  plates,  and  masses  produced  by 
the  cells  either  within  the  cell-wall  or  outside  of  it.     The  tendons 
which  unite  muscles  to  bones,  and  the  bones  themselves,  illus- 
trate the  two  kinds  of  tissues  in  this  group. 

(3)  Muscular  tissues  (Fig.  48  D,  E)  are  the  agents  of  active 
movement.     We  found  (Chap.  IV)  that  the  locomotion  of  Ameba 
could  be  explained  by  the  presence  of  contractile  fibers.     In  other 
Protozoons  belonging  to  the   Classes  Infusoria  and  Sporozoa 
there  are  muscular  fibrils  called  myonemes  in  the  membranous 
coverings.     In  most  of  the  higher  organisms  special  muscle  cells 
are  differentiated  for  performing  the  various  movements  of  the 
body.     These  cells  possess  muscle  fibrils  which  are  able  to  con- 
tract with  great  force  and  in  quick  succession.     The  fibrils  are 
usually  of  two  kinds;  (a)  cross  striated  (D)  and  (b)  smooth  non- 
striated  (E).     The  latter  form  a  less  highly  developed  tissue  than 
the  former  and  are  found  in  the  simpler  inactive  animals,  and 
in  those  internal  organs  of  higher  organisms  not  subject  to  the 
will  of  the  animal. 

(4)  Nervous  tissue  is  composed  of  cells  which  are  so  acted 
upon  by  external  physical  and  chemical  agents  that  they  are 
able  to  perceive  a  stimulus,  to  conduct  it  to  some  other  cell  or 
cells  of  the  body,  and  to  stimulate  still  other  cells  to  activity. 
All  protoplasm  is  irritable;    animals  without  nervous  systems, 
e.g.  Ameba,  are  capable  of  reacting  to  a  stimulus,  but  in  more 
complex  organisms  certain  cells  are  specialized  for  the  sole  pur- 


102  AN  INTRODUCTION  TO  ZOOLOGY 

pose  of  performing  the  functions  described  above  as  characteristic 
of  nervous  tissue. 

2.  THE  GERM  CELLS 

We  have  seen  that  the  substance  of  which  all  animals  are  com- 
posed may  be  separated  into  reproductive  and  non-reproductive. 
The  latter  is  called  somatic-plasm,  the  former  germ-plasm.  No 
organism  arises  fully  developed,  but  usually  originates  from  a 
union  of  two  germ  cells  which  have  been  produced  by  a  preexist- 
ing organism.  There  is  a  gradual  development  of  an  individual 
from  the  fertilized  germ  cell,  the  organism  passing  through  a 
number  of  well-defined  stages  before  the  adult  condition  is  at- 
tained. To  bring  the  various  steps  of  this  process  clearly  before 
us  we  shall  now  discuss  the  origin  and  ripening  of  the  germ  cells, 
their  union  in  fertilization,  and  the  subsequent  developmental 
history  of  the  zygote  thus  formed.  These  subjects  will  be  con- 
sidered in  some  detail  for  each  type  studied  in  the  following  chap- 
ters, so  we  shall  here  give  only  a  general  outline  which  will  serve 
as  a  foundation  for  future  discussions. 

Early  in  the  history  of  the  individual  certain  cells  are  set  aside 
for  the  sole  purpose  of  reproduction;  these  are  the  germ  cells. 
All  the  other  cells  of  the  developing  organism  become  more  or 
less  specialized  for  the  various  vegetative  functions,  and  may 
eventually  be  recognized  as  nerve  cells,  muscle  cells,  etc.  Only 
the  germ  cells  remain  in  their  primitive  condition,  and,  since 
they  take  no  part  in  the  daily  life  of  the  animal,  we  may  think  of 
them  as  being  passively  carried  about  and  protected  by  the  soma- 
tic cells.  Just  before  the  individual  becomes  sexually  mature,  the 
germ  cells  awake  to  activity  and  a  number  of  complex  processes 
are  inaugurated  which  result  in  the  casting  out  of  certain  of  them 
by  the  body  to  produce  new  individuals  similar  to  the  parent 
organism. 

Spermatogenesis.  —  The  development  of  the  male  germ  cell  or 
spermatozoon  is  termed  spermatogenesis.  As  shown  in  Figure  49, 
this  process  maybe  divided  into  three  periods:  (i)  the  multiplica- 


AN  INTRODUCTION  TO  THE  METAZOA 


103 


tion  of  the  primordial  germ  ceils  or  spermatogonia,  (2)  the 
growth  of  these  cells,  and  (3)  their  ripening  or  maturation. 
These  stages  occur  in  all  Metazoa,  from  the  lowest  to  man.  No 
one  knows  how  many  cells  are  produced  during  the  period  of 
multiplication.  The  last  generation  of  spermatogonia  gives 


PRIMORDIAL 
GERM-CELL 


BPERMATOGONIA^.-' 


PRIMARY 
SPERMATOCYTE 


SECONDARY 
SPERMATOCYTES 


SPERMATIDS 


SPERMATOZOA 


MULTIPLICATION 
PERIOD 


GROWTH 
PERIOD 


MATURATION 
PERIOD 


FIG.  49.  Diagram  illustrating  the  stages  of  spermatogenesis.     The  primor- 
dial germ  cell  is  represented  as  possessing  four  chromosomes. 

rise  by  division  to  the  primary  spermatocytes.  The  latter 
increase  greatly  in  size  during  the  long  growth  period,  and  in 
each  of  them  the  chromosomes  unite  or  conjugate  to  form 
double  or  bivalent  chromosomes.  Each  primary  spermatocyte 
gives  rise  by  division  to  two  secondary  spermatocytes.  During 
this  division  the  chromosomes,  which  united  to  form  the  bivalent 
chromosomes,  separate,  one  single  or  univalent  chromosome 
going  to  each  secondary  spermatocyte.  This  is  the  only  known 
case  in  cell  division  where  entire  chromosomes  are  separated 
from  one  another,  except  the  corresponding  stage  in  oogenesis. 
It  is  known  as  a  reduction  division  because  it  results  in  a  re- 
duction in  the  number  of  chromosomes  to  one  half  in  the 
daughter  cells.  The  secondary  spermatocytes  immediately 


io4 


AN  INTRODUCTION  TO  ZOOLOGY 


divide,  each  forming  two  spermatids.  Each  spermatid  receives 
one  half  of  each  chromosome  from  the  secondary  spermatocyte 
that  gives  rise  to  it.  The  spermatids  are  then  metamorphosed 
into  spermatozoa. 

The  spermatozoa  of  various  animals  are  usually  easily  dis- 
tinguished one  from  another,  but  are  mostly  constructed  on  the 


PRIMORDIAL 
GERM-CELL 


ULTIPLICATION 
PERIOD 


PRIMARY 
OOCYTE 


SECONDARY 

OOCYTES 
(OVARIAN  EGG 
AND  POLAR.BODY) 

MATURE  EGG 

AND  — 

POLAR  BODIES 


FIG.  50.  Diagram    illustrating    the    stages   of   oogenesis.    The    primordial 
germ  cell  is  represented  as  possessing  four  chromosomes. 

same  plan.  They  resemble  an  elongated  tadpole,  having  a  head 
filled  almost  entirely  with  nuclear  material  and  a  long  flagellum  • 
like  tail,  which  is  the  organ  of  locomotion;  the  middle  piece 
joining  these  two  is  the  centrosome.  The  spermatozoa  are 
the  active  germ  cells;  it  is  their  duty  to  seek  out  and  fertilize 
the  larger  stationary  egg  cells.  Frequently  they  are  only 
•nr^Tnnr  the  size  of  the  egg,  and  in  the  sea  urchin,  Toxopneustes, 
their  bulk  is  about  -^-frWo  the  volume  of  the  ovum  (99). 

Oogenesis.  —  The  origin  of  the  egg  is  called  oogenesis  (Fig.  50). 
Stages  are  passed  through  by  the  germ  cells  corresponding  al- 
most exactly  to  those  described  under  spermatogenesis  (Fig.  49). 
Before  the  growth  period  the  germ  cells  which  will  produce  eggs 
are  known  as  oogonia  (Fig.  50;  Fig,  51,  a).  At  the  completion 


AN  INTRODUCTION  TO  THE  METAZOA 


105 


of  the  growth  period  they  are  termed  primary  oocytes  (Fig.  51,  b). 
The  primary  oocytes  contain  only  one  half  the  number  of  chromo- 
somes characteristic  of  the  somatic  cells  and  oogonia.  As  in 
the  primary  spermatocytes,  these  chromosomes  are  bivalent. 


p.b.2 


FIG.  51.  Diagrams  illustrating  the  maturation,  fertilization,  and  cleavage 
of  an  egg.  The  primordial  germ  cell  is  represented  as  possessing  four 
chromosomes. 


106  AN  INTRODUCTION  TO  ZOOLOGV 

resulting  from  the  union  two  by  two  of  the  univalent  chromosomes 
of  the  oogonia.  The  primary  oocyte  divides  in  the  following 
manner.  Its  nucleus,  called  the  germinal  vesicle  (Fig.  51,  a), 
moves  to  the  periphery  (b)  where  a  mitotic  figure  is  formed  per- 
pendicular to  the  surface  of  the  egg  (c).  A  small  bud-like  protru- 
sion is  now  formed  into  which  pass  one  univalent  chromosome 
from  each  of  the  bivalent  chromosomes  present  in  the  primary 
oocyte  (d).  The  bud  is  then  pinched  off.  Two  secondary  oocytes 
are  produced  by  this  division,  each  containing  an  equal  amount  of 
chromatin,  but  one  with  a  great  deal  more  cytoplasm  and  yolk 
than  the  other  (e).  The  small  one  is  known  as  the  first  polar 
body  (e,  p.  b.  i)  and  is  not  functional;  the  larger  is  the  egg.  Each 
secondary  oocyte  now  prepares  for  division  (/).  The  first  polar 
body  in  some  cases  does  not  divide;  when  it  does  the  division  is 
equal  (g,  p.  b.  i).  The  egg  throws  off  a  second  polar  body  (g, 
p.  b.  2)  which  contains  one  half  of  each  chromosome.  This 
second  polar  body  disintegrates,  as  does  the  first. 

Fertilization.  —  The  mature  ovum  now  becomes  the  center  of 
the  interesting  process  of  fertilization.  The  spermatozoon 
sometimes  enters  the  egg  before  the  polar  bodies  are  formed,  and 
sometimes  afterward.  In  the  illustrations  (Fig.  $i,/)  we  have 
shown  the  sperm  entering  the  egg  at  the  end  of  the  first  oocyte 
division.  The  sperm  brings  into  the  egg  a  nucleus,  a  centro- 
some,  and  a  very  small  amount  of  cytoplasm.  The  sperm  nucleus 
soon  grows  larger  by  the  absorption  of  material  from  the  cyto- 
plasm of  the  egg,  and  the  centrosome  begins  its  activity.  A  mi- 
totic figure  soon  grows  up  (g)  and  moves  toward  the  center  of  the 
egg.  The  egg  nucleus  also  moves  in  this  direction  (h) ,  and  finally 
both  the  male  and  female  nuclei  are  brought  together  in  the  midst 
of  the  spindle  produced  about  the  sperm  nucleus  (i).  This  com- 
pletes the  process  usually  known  as  fertilization.  In  this  process 
the  chief  aim  so  far  seems  to  be  the  union  of  two  nuclei,  one  of 
maternal  origin,  the  other  of  paternal  origin.  We  shall  see  later 
that  fertilization  is  really  not  consummated  until  the  animal 
which  develops  from  the  egg  has  become  sexually  mature. 


AN  INTRODUCTION  TO  THE  METAZOA  107 

Chromosome  Reduction.  —  We  are  able  to  explain  now  why  a 
reduction  in  the  number  of  chromosomes  takes  place  during 
maturation.  It  has  already  been  pointed  out  (p.  32)  that 
every  species  of  animal  has  a  definite,  even  number  of  chromo- 
somes in  its  somatic  cells.  This  number  remains  constant  gener- 
ation after  generation.  Now  if  the  mature  egg  contained  this 
somatic  number  of  chromosomes  and  the  sperm  brought  into  it  a 
like  number,  the  animal  which  developed  from  the  zygote  would 
possess  in  its  somatic  cells  twice  as  many  as  its  parents.  The 
number  is  kept  constant  by  reduction  during  the  maturation 
divisions,  so  that  both  egg  and  sperm  contain  only  one  half  the 
number  in  the  somatic  cells.  The  union  of  egg  and  sperm  again 
establishes  the  normal  number  of  chromosomes  possessed  by  the 
parents. 

Union  of  Chromosomes  in  Fertilization.  —  If  we  return  for  a 
moment  to  the  subject  of  maturation,  the  final  process  in  fertiliza- 
tion may  be  understood.  It  appears  that  chance  has  very  little  to 
do  with  the  union  of  chromosomes  in  pairs  during  the  early  his- 
tory of  the  germ  cells  (pp.  103  106,  Figs.  49,  50,  51,  b);  but  that 
one  chromosome  of  each  pair  came  originally  from  the  egg  and  is 
therefore  maternal,  while  the  other  was  derived  from  the  sperm 
and  is  paternal.  Since  the  chromosomes  are  recognized  as  the 
bearers  of  hereditary  qualities  (p.  32)  it  follows  that  the  blend- 
ing of  the  characteristics  of  the  mother  and  father  in  the  germ 
cells  does  not  occur  when  the  sperm  enters  the  egg,  but  when  the 
individual  developing  from  the  zygote  becomes  sexually  mature. 

3.  EMBRYOLOGY 

Cleavage.  —  The  division  of  the  fertilized  egg  is  known  as 
cleavage.  The  chromatin  of  the  united  germ  nuclei  condenses 
into  chromosomes,  which  are  so  arranged  on  the  first  cleavage 
spindle  (Fig.  51,7)  that  each  daughter  nucleus  receives  half  of 
each.  This  means  that  each  daughter  cell  will  contain  half  of 
each  chromosome  of  paternal  origin  and  half  of  each  chromosome  of 


108  AN  INTRODUCTION  TO  ZOOLOGY 

maternal  origin.  Further  mitotic  divisions  insure  a  like  distribu- 
tion to  every  cell  in  the  body.  After  nuclear  division  comes  the 
division  of  the  entire  cells  into  two  (k  and  /). 

Typically  the  fertilized  egg  divides  into  two  cells,  these  two 
into  four,  these  four  into  eight,  etc.,  each  cleavage  plane  being 
perpendicular  to  the  last  preceding  plane  (Fig.  53).  This  is 
known  as  total  cleavage,  and  is  characteristic  of  holoblastic  eggs. 
Other  eggs  are  said  to  be  meroblastic  because  they  exhibit  partial 
cleavage,  that  is,  only  a  small  part  of  the  egg  enters  into  cell  divi- 
sion, the  remainder  serving  as  nutritive  material  for  the  cleavage 
cells.  In  all  we  can  recognize  four  distinct  types  of  cleavage: 
(i)  equal  cleavage,  where  the  egg  divides  into  two  equal  halves 
(Fig.  52,  A);  (2)  unequal  cleavage,  where  the  first  division  of  the 
egg  results  in  one  large  and  one  small  cell  (Fig.  52,  B);  (3)  dis- 
coidal  cleavage,  where  the  entire  egg  does  not  divide,  but  small 
cells  are  cut  off  at  the  surface  and  form  a  disk-shaped  region 
(Fig.  52,  C);  and  (4)  superficial  cleavage,  where  the  nucleus  of  the 
egg  divides  rapidly;  the  daughter  nuclei  migrate  to  the  periphery 
and  form  a  single  layer  of  cells  at  the  surface  (Fig.  52,  D). 

That  part  of  ontogeny  which  concerns  the  development  of  an 
animal  from  the  egg  to  maturity  is  known  as  embryogeny.  Cer- 
tain stages  in  this  development  have  been  recognized  as  common 
to  all  higher  animals,  and  have  been  given  names.  The  stages 
occur  in  a  certain  regular  order,  and  as  an  introduction  to  the  more 
detailed  special  accounts  given  for  each  type  studied  in  subse- 
quent chapters,  we  shall  present  a  brief  embryological  history  of  a 
typical  holoblastic  egg.  The  stages  to  be  considered  are:  (i) 
cleavage,  (2)  the  morula,  (3)  the  blastula,  (4)  the  gastrula,  (5)  the 
formation  of  germ  layers,  and  (6)  organogeny. 

•  Cleavage  in  a  holoblastic  egg  (Fig.  53)  results  in  the  production 
of  two  (B),  four  (C,  D),  eight  (E),  sixteen  (F),  etc.,  cells  approxi- 
mately equal  to  one  another  and  growing  smaller  as  their  number 
increases.  Each  of  these  cells  is  known  as  a  blastomere.  The 
blastomeres  do  not  separate  as  do  the  daughter  cells  produced 
by  the  binary  division  of  Paramecium  (Fig.  31,  o-q)  but  remain 


FIG.  52.  Figures  illustrating  four  different  kinds  of  cleavage.  A,  equal 
cleavage  of  the  sea  urchin  egg;  B,  unequal  cleavage  of  the  egg  of 
marine  worm  ;  C,  discoidal  cleavage  of  the  egg  of  a  squid  ;  D,  superficial 
cleavage  of  an  insect's  egg.  (A-B,  from  Wilson  ;  C,  from  Wilson  aftei 
Watase;  D,  from  Korschelt  and  Heider.) 


o 


FIG.  53.  Figures  illustrating  the  cleavage  of  a  holoblastic  egg,  and  the  formation  of  germ 
layers.  A-K,  cleavage  and  formation  of  the  blastula  ;  L-M,  gastrulation  ;  N,  pro- 
duction of  the  mesoderm  and  ccelomic  cavities ;  O,  coelom  further  developed ;  ak, 
ectoderm;  dh,  primitive  alimentary  canal;  ik,  entoderm  ;  mk\,  somatic  layer  of 
mesoderm  ;  mkz,  splanchnic  layer  of  mesoderm.  (From  Korschelt  and  Heider  after 
Hatschek.) 


110  AN  INTRODUCTION  TO  ZOOLOGY 

attached  to  one  another  as  was  noted  in  the  case  of  Pandorina 
(p.  94,  Fig.  45,  II).  The  resemblance  of  the  group  of  blas- 
tomeres  to  a  mulberry  suggested  the  term  morula,  which  is  often 
used  in  describing  the  egg,  during  the  early  cleavage  stages. 

Blastula.  —  As  cleavage  advances,  a  cavity  becomes  noticeable 
in  the  center  of  the  egg  (Fig.  53,  H),  becoming  larger  as  develop- 
ment proceeds  until  the  whole  resembles  a  hollow  rubber  ball, 
the  rubber  being  represented  by  a  single  layer  of  cells.  At  this 
stage  the  egg  is  called  a  blastula,  the  cavity  the  cleavage  or  seg- 
mentation cavity,  and  the  cellular  layer  the  blastoderm,  The 
blastula  resembles  somewhat  a  single  colony  of  Volvox  (Fig. 
46,  A). 

Gastrula.  —  The  cells  on  one  side  of  the  blastula  are  seen  to  be 
thicker  than  elsewhere  (Fig.  53,  K)  and  begin  to  invaginate 
(Fig.  53,  L).  This  process  results  in  a  cup-shaped  structure  with 
a  wall  of  two  layers,  an  outer  layer  of  small  cells  and  an  inner 
layer  of  larger  cells.  The  embryo  may  now  be  called  a  gastrula 
(M),  and  the  process  by  wHich  it  developed  from  the  blastula 
is  termed  gastrulation.  The  cleavage  cavity  is  almost  obliterated 
during  the  invagination,  while  a  new  cavity,  the  primitive  diges- 
tive tract  or  archenteron,  is  established. 

Germ  layers.  —  The  cells  of  one  layer  of  the  gastrula  resemble 
one  another,  but  differ  in  appearance  from  the  cells  of  the  other 
layer.  Each  layer  gives  rise  to  certain  definite  parts  of  the  body, 
and  is  therefore  termed  a  germ  layer;  the  outer  is  the  ectoderm 
(Fig.  53,  N,  ak.),  the  inner,  the  entoderm  (N,  ik.).  Animals  with 
only  these  two  layers  are  said  to  be  diploblastic;  but  the  majority 
of  the  higher  animals  have  a  third  layer,  which  usually  appears 
between  the  first  two  after  the  gastrula  has  been  formed.  This  is 
the  middle  layer,  or  mesoderm.  It  originates  either  from  the  pro- 
liferation of  a  few  special  cells  which  may  be  recognized  in  the 
early  cleavage  stages,  or  from  cells  budded  off  from  the  inner 
surface  of  both  the  ectoderm  and  entoderm,  or  from  pouches 
arising  from  the  walls  of  the  entoderm  (Fig.  53,  N).  Animals 
with  three  germ  layers  are  said  to  be  triploblastic. 


AN  INTRODUCTION  TO  THE  METAZOA  in 

Organogeny.  —  Each  germ  layer  gives  rise  to  certain  organs 
of  the  adult  animal.  An  organ  may  be  defined  as  an  association 
of  tissues  having  a  definite  form  and  special  function.  In  any 
of  the  more  complex  animals  eight  systems  of  organs  can  be  recog- 
nized: (i)  digestive,  (2)  circulatory,  (3)  respiratory,  (4)  excre- 
tory, (5)  skeletal  and  integumentary,  (6)  reproductive,  (7)  mus- 
cular, (8)  nervous.  The  study  of  the  origin  of  these  organs  from 
the  germ  layers  is  known  as  organogeny.  The  organs  derived 
from  the  different  germ  layers  may  be  briefly  listed  as  follows. 
From  the  ectoderm  arise  the  epidermis,  epithelium  of  various 
organs,  and  the  nervous  system;  from  the  mesoderm  come  the 
muscles,  connective  and  supporting  tissues  and  blood  and  blood 
vessels;  the  entoderm  becomes  the  epithelium  of  the  digestive 
tract,  pharynx,  and  respiratory  tract. 

Ccelom.  —  Figure  53,  N,  O  shows  one  method  of  origin  of  the 
coelom,  a  cavity  of  great  importance  in  the  bodies  of  many  of  the 
complex  Metazoa.  Two  pouches  (N)  formed  from  the  inner 
layer  of  the  gastrula  are  pinched  off  and  come  to  lie  one  on  either 
side  of  the  alimentary  canal  (O).  The  cavities  of  these  sacs 
constitute  the  ccelom.  By  a  lateral  growth  and  a  breaking 
through  at  the  ends  of  the  sacs,  the  cavities  unite  to  form  a  single 
space  between  the  alimentary  canal  and  the  body  wall.  All 
Metazoons  are  frequently  separated  into  two  groups,  the  ccelo- 
mata  and  the  acalomata,  according  as  the  ccelom  is  present  or 
absent.  Of  the  types  considered  in  the  following  chapters 
Hydra  may  be  mentioned  as  an  accelomate,  the  earthworm  as 
a  ccelomate. 

Table  III  contains  a  series  of  diagrams  and  descriptions  which 
are  intended  to  represent  the  methods  of  reproduction  exhibited 
by  the  types  already  studied,  and  to  show  those  of  certain  Meta- 
zoa as  well.  In  order  to  make  the  diagrams  as  simple  as  possible 
certain  details  have  beeen  omitted;  nevertheless,  it  is  believed 
that  a  correct  idea  of  the  reproductive  processes  will  be  gained 
in  every  case. 


112 


AN  INTRODUCTION  TO  ZOOLOGY 


TABLE  III 


DIAGRAMS    ILLUSTRATING    THE    METHODS    OF    REPRODUCTION   IN    PROTOZOA 

AND  METAZOA 


A.   A  simple  Protozoon  (Ameba). 


psp 


CELL  DIVISION 

(Binary  Fission) 

An  indefinite  number  of 

generations. 


ENCYSTMENT  AND  SUBSE- 
QUENT CELL  DIVISION 

(Sporulation) 

Many  pseudopodiospores  (psp) 
'  (young  Amebce)  produced. 


CELL  DIVISION 

(Binary  Fission) 

An  indefinite  numbei 

of  generations. 


B.   A  simple  Protozoon  (Paramecium). 


CELL  DIVISION      TEMPORARY  CONJU- 
( Binary  Fission)  CATION 

An  indefinite  num-        (Fertilization) 
her  of  generations.     Each  cell  fertilizes 
the  other. 


CELL  DIVISION        CELL  DIVISION 
(Period  of  Recon-     (Binary  Fission) 
struction)  An  indefinite  num 

Each  fertilized  cell  her  of  generations, 
gives  rise  to  four 
normal  animals. 


AN  INTRODUCTION  TO  THE  METAZOA 


C.   A  simple  Protozoon  (Plasmodium) . 


CELL  DIVISION       CELL  DIVISION        PERMANENT  CON-  CELL  DIVISION  CELL  DIVISION 

(Schizogany)         (Gametogenesis)               JUGATION  (Sporogany)      (Schizogany) 

A  sporozoite  (sz)    Certain  merozoites         (Fertilization)  A  zygote  gives     A  sporozoite 

produces  many     divide,  forming  one     A  female  cell  (egg)  rise  to  many    produces  many 

merozoites  (mz).  egg  (e}  and  one  polar  fuses  with  a  male  sporozoites(sz).  merozoites (mz). 
body  (pb)\  others  give    cell  (sperm}  pro- 
rise  by  division  to  as     ducing  a  zygote 
many  as  eight  sper-               (a), 
matozoa  (s/>). 


D.   A  simple  colony  of  cells  (Pandorina). 


CELL  DIVISION  CLLL  DIVISION  CELL  DIVISION  CELL  DIVISION   PERMANENT  CELL  DIVISION 

(Swarm  spore      (Colony  for-       (Colony  for-      (Gamete  for-     CONJUGATION  (  Swarm  spore 

formation)  mation)  mation)  mation)          The  gametes      formation) 

Zygote  gives       Each  swarm     Each  cell  of  the  Each  cell  of  the  unite  in  pairs,  Zygote  gives 


rise  to  from  i    spore  produces      colony  pro- 

to  3  swarm       a  new  colony,     duces  a  new 

spores.  colony. 


colony  pro- 
duces a  colony 
of  gametes 
which  separate 
from  one 
another. 


forming         rise  to  i  to  3 
zygotes.       swarm  spores. 


AN  INTRODUCTION  TO  ZOOLOGY 


E.   A  more  complex  colony  of  cells  (Volvox). 


CELL  DIVISION  CELL  DIVISION  CELL  DIVISION    PERMANENT  CELL  DIVISION 
(Colony  for-       (Asexual  Re-       (Gamete  for-      CONJUGATION  (Colony  for- 
mation)          production)            mation)         (Fertilization)  mation) 
Zygote  (2)      Germ  cells  (g.c.)    Certain  germ        One  sperm  Zygote  develops 
develops  into  a      give  rise  to        cells  produce    fuses  with  one  into  a  colony. 
colony.           new  colonies,    eggs(e);  others  egg,  forming  a 
spermatozoa        zygote  (z). 


F.   A  simple  Metazoon  (Hydra). 


CELL  DIVISION 
( Embryological 
development) 
Zygote  (z)  pro- 
duces animal 

containing 

germ  cells  (g.c.) 

and  two  layers 

of  specialized 

somatic  cells,  the 

ectoderm  (ec.) 

and  entoderm 

(*»•). 


BUDDING       CELL  DIVISION    PERMANENT     CELL  DIVISION 
(Asexual          (Gamete  for-     CONJUGATION    (Embryological 
Reproduction)         mation)         (Fertilization)     development) 
Part  of  animal     Certain  germ        One  sperm      Zygote  (z)  pro- 
separates  from     cells  produce     uniteswith  one    duces  animal, 

parent  and      one  egg  (e)  and  egg,  forming  a  etc. 

leads  separate         two  polar  zygote  (z). 

existence.  bodies  (ph.); 
others  produce 
many  sperms 


AN  INTRODUCTION  TO  THE  METAZOA 


G.   A  complex  Metazoon  (Lumbricus). 

z  g.c.  pb.e 


CELL  DIVISION 
(Embryological 
development) 
Zygote  (2)  pro- 
duces body 
containing 
germ  cells  (g.c.) 
and  three  layers 
of  specialized 
somatic  cells, 
the  ectoderm 
(ec.),  mesoderm 

(ws.),  and 
entoderm  (en.). 


CELL  DIVISION 
(Gamete  for- 
mation) 
Certain  germ 
cells  produce 
one  egg  (<•)  and 

two  polar 

bodies  (pb.); 

others  produce 

many  sperms 


PERMANENT 
CONJUGATION 
(Fertilization) 
One  sperm 
unites  with 
one  egg  form- 
ing a  zygote 


CELL  DIVISION 
( Embryological 
development) 
Zygote  (2)  pro- 
duces body,  etc. 


CHAPTKR   VIII 


HYDRA   AND    CGELENTERATES   IN    GENERAL 
i.  HYDRA 

(Hydra  fusca  Linnaeus) 

Hydra  fusca  is  a  simple  Metazoon  abundant  in  fresh  water 
ponds  and  streams.  If  a  quantity  of  aquatic  vegetation  is  gat  h 
ered  and  placed  in  glass  dishes  full  of  water,  these  little  fresh-water 
polyps  will  be  found  clinging  to  the  plants  and  the  sides  and  bot- 
tom of  the  dish.  They  are  easily  seen  with  the  naked  ryr.  bring 
from  2  to  20  mm.  in  length,  and  may  be  likened  to  a  short  ihirk 
thread  frazzled  at  the  unattached,  distal  end.  The  great  varia- 
tion in  length  is  due  to  the  fact  that  both  body  and  trnlarlrs  air 
capable  of  remarkable  expansion  and  contraction  because  of  the 
presence  of  specialized  muscle  fibrils  in  many  of  the  cells. 

TABLE  IV 

THE  NAMES  AND  CHARACTERISTICS  OF  THE  PRINCIPAL  SPECIES  OF  HYDRA  (105) 


C.VM  S 

SIM  (IKS 

COLOR 

TEN- 

1  \rns 

\.M\ 

IOCYSI  s 

SEXUAL 
CONDITION 

1  i  \.n  11 

or  BODY 

W.IKN  Svx- 

i  \\\\ 
MAIT  KI 

Hydra 

fusca 

brown 

10-6 
or  fewer 

large 

Hermaphro- 
ditic 

2  cm. 

Sept.-Oct. 

« 

viridis 

green 

5-12 

small 

« 

1-1.5  cm, 

Apr.  -Oct. 

« 

grisea 

gray 

5-i8 

very 

« 

2  cm. 

Apr.-Aug. 

large 

sometimes 

to  Her. 

«< 

didfda 

pale 

5-8 

sexes 

2.5  cm. 

Oct.-Dec. 

yellow 

distinct 

or 

reddish 

brown 

116 


HYDRA  AND  CCELENTERATES  IN  GENERAL          117 

A  number  of  species  of  Hydras  are  recognized  by  zoologists; 
some  of  the  characteristics  of  the  principal  ones  are  given  in 
Table  IV.  Hydra  fusca  has  been  selected  as  a  type  because  it  is 
the  species  most  easily  obtained  for  laboratory  use. 

External  Characters.  —  The  body  of  Hydra  fusca  resembles  an 
clastic  tube  which  varies  in  length  and  thickness  according  as 
the  animal  is  extended  or  contracted;  in  the  former  case  it  may 
reach  a  length  of  2  cm.  At  the  distal  end  is  a  circlet  of  from  six  to 
ten  slender,  finger-like  projections  called  tentacles.  The  diameter  of 
the  body  is  frequently  increased  at  certain  points  by  a  distention 
due  to  the  ingestion  of  large  particles  of  food.  The  general  color 
is  brown.  Usually  two  regions  may  be  noted;  a  thin,  nearly 
colorless  portion  just  below  the  tentacles  and  a  thicker  and  more 
deeply  colored  part  extending  to  the  opposite  pole.  The  part  of 
the  body  which  is  usually  attached  to  some  object  is  known  as 
the  foot  or  Ixisdl  disk  and  is  referred  to  as  the  proximal  end.  The 
foot  not  only  anchors  the  animal  when  at  rest,  but  also  serves  as  a 
locomotor  organ.  A  conical  elevation,  the  hypostome,  occupies 
the  distal  end  of  the  body.  It  is  surrounded  by  the  tentacles, 
and  has  at  the  top  an  opening,  the  mouth.  This  mouth  is  not 
the  simple  circular  orifice  often  described,  but  is  star-shaped, 
having  clefts  running  out  from  the  center  toward  each  arm 
(104). 

The  tentacles  are  capable  of  remarkable  expansion,  and  may 
stretch  out  from  small  blunt  projections  to  very  thin  threads 
7  cm.  or  more  in  length;  in  this  condition  they  are  so  thin  as  to 
be  barely  visible  even  with  a  lens.  They  move  independently, 
capturing  food  and  bringing  it  into  the  mouth.  Their  number 
varies  as  shown  in  Table  IV.  Reese  (116)  examined  six  hundred 
specimens  of  Hydra  riridis  and  found  from  four  to  twelve  ten- 
tacles on  each.  These  occurred  in  the  following  proportions: 
54  per  cent  had  eight;  24  per  cent,  seven;  15  per  cent,  nine; 
very  u\\  animals  possessed  a  greater  number  than  nine,  and  only 
occasionally  was  one  found  with  less  than  seven.  The  number  of 
tentacles  increase's  with  the  size  and  age  of  the  animal,  although 


Ji8 


AN  INTRODUCTION  TO  ZOOLOGY 


unfavorable  conditions  and  extreme  age  result  in  a  decrease 
(116). 

Frequently  specimens  of  Hydra  are  found  which  possess  buds 
in  various  stages  of  development.  Several  buds  are  often  found 
on  a  single  animal,  and  these  in  tui  n  may  bear  buds  before  detach- 


g 


..b 


l.d. 


FIG.  54.  Diagram  of  a  longitudinal  section  of  Hydra;  b.,  bud ;  b.d.,  basal 
disk;  bl.,  blastula ;  ec.,  ectoderm;  en.,  entoderm ;  g.,  gastrula ;  gv.c., 
gastrovascular  cavity  ;  hy.,  hypostome  ;  m.,  mouth  ;  m.e.,  mature  egg  ; 
mes.,  mesoglea;  m.t.,  mature  testis  ;  n.,  nematocysts  ;  p.b.,  polar  bod- 
ies ;  /.,  tentacle  ;  y.e.,  young  egg  ;  y.t.,  young  testis.  All  of  the  struc* 
tures  shown  do  not  occur  on  a  single  animal  at  one  time. 


HYDRA  AND   CCELENTERATES  IN   GENERAL          ng 

ment  from  the  parent.  In  this  way  a  sort  of  primitive  Hydra 
colony  is  formed,  resembling  somewhat  the  asexual  colonies  of 
some  of  the  more  complex  Ccelenterates  to  be  described  later 
(Fig.  65,  A). 

REPRODUCTIVE  ORGANS  may  be  observed  on  specimens  of 
Hydra  fusca  in  September  and  October.  Both  an  ovary  and  testes 
are  produced  on  a  single  individual ;  the  former  is  knob-like,  occupy- 
ing a  position  about  one  third  the  length  of  the  animal  above  the 
basal  disk;  the  testes,  usually  two  or  more  in  number,  are  conical 
elevations  projecting  from  the  distal  third  of  the  body. 

Structure  (Fig.  54).  —  Hydra  is  a  diploblastic  animal  con- 
sisting of  two  cellular  layers,  an  outer  thin  colorless  layer,  the 
ectoderm  (ec.),  and  an  inner  layer,  the  entoderm  (en.),  twice  as 
thick  as  the  outer  and  containing  the  brown  bodies  which  give 
Hydra  fusca  its  characteristic  color.  Both  layers  are  composed 
of  epithelial  cells.  A  thin  space  containing  a  jelly-like  substance, 
the  mesoglea  (mes.)  separates  ectoderm  from  entodeim.  Not 
only  the  body  wall,  but  also  the  tentacles,  possess  these  three 
definite  regions.  The  body,  with  the  exception  of  the  basal  disk, 
is  covered  by  a  thin  transparent  cuticle.  Both  body  and  ten- 
tacles are  hollow,  the  single  central  space  being  known  as  the 
gastrovascular  cavity  (gv.c.). 

ECTODERM.  —  The  ectoderm  is  primarily  protective  and  sensory, 
containing  structures  characteristic  of  these  functions.  Slight 
differences  in  structure  are  observable  between  the  ectoderm  of 
the  tentacles  and  that  of  the  body  wall,  while  the  latter  differs 
from  that  of  the  basal  disk.  In  the  ectoderm  of  the  body  wall 
are  two  principal  kinds  of  cells,  large  epitheliomuscular  cells, 
and  small  interstitial  cells.  The  latter  give  rise  to  cells  called 
cnidoblasts  which  form  nematocysts,  and  to  both  male  and 
female  germ  cells.  The  epitheliomuscular  cells  are  shaped  like 
inverted  cones.  At  their  inner  ends  are  one  or  more  compara- 
tively long  (sometimes  .38  mm.)  unstriped  contractile  fibers 
which  form  a  thin  longitudinal  muscular  layer.  These  muscle 
fibers  explain  the  remarkable  powers  of  contraction  exhibited 


120 


AN  INTRODUCTION  TO  ZOOLOGY 


by  Hydra  when  stimulated.  Near  the  middle  of  each  cell,  em- 
bedded in  the  alveolar  cytoplasm,  is  a  nucleus  containing  one  or 
two  nucleoli  and  a  network  of  chromatin. 

NEMATOCYSTS  or  stinging  capsules  (Figs.  54,  n ;  56)  are  present 
on  all  parts  of  the  body  of  Hydra  except  the  basal  disk,  being 

most  numerous  on 
the  tentacles  (Fig. 
5 5,  A).  Each  is  con- 
tained in  a  cell 
known  as  a  cnido- 
blast.  These  in  turn 
are  embedded  in  lit- 
tle tubercles  on  the 
surface  which  give 
the  animal  a  rough- 
appearing  outline. 
The  tubercles  are 
ectoderm  cells,  each 
of  which  usually  pos- 
sesses one  or  more 
large  nematocysts 
surrounded  by  a 
number  of  a  smaller 
variety.  Three  kinds 
of  nematocysts  are 
found  in  Hydra.  The 
largest  is  .013  mm. 
long  and  .007  mm.  thick ;  before  being  discharged  it  is  pear-shaped 
and  occupies  almost  the  entire  cell  in  which  it  lies  (Fig.  56,  nem.). 
Within  it  is  &  coiled  tube  (t)  at  whose  base  are  three  large  and  a  num- 
ber of  small  spines.  Projecting  from  the  cell  near  the  outer  end  of 
the  nematocyst  is  a  trigger-like  spine,  the  cnidocil  (cm.).  Nema- 
tocysts may  be  exploded  by  adding  a  little  acetic  acid,  or  better, 
methyl  green,  to  the  water.  The  tube  which  is  coiled  within 
them  is  then  everted.  First,  the  base  of  the  tube  with  the  spines 


FIG.  55.  Nematocysts  and  their  action.  A,  por- 
tion of  a  tentacle  showing  the  batteries  of 
nematocysts ;  cl.,  cnidocils ;  B,  insect  larva 
covered  with  nematocysts  as  a  result  of  cap- 
ture by  Hydra.  (From  Jennings.) 


HYDRA  AND   CCELENTERATES  IN   GENERAL 


121 


cnc 


appears,  and  then  the  rest  of  the  tube  rapidly  turns  inside  out. 
Nematocysts  are  able  to  penetrate  the  tissues  of  other  animals, 
but  only  at  their  greatest  speed  and  before  eversion  is  completed. 
Even  the  extremely  firm  chitinous  covering  of  insects  may  be 
punctured  by  these  structures  (Figs.  55,  B  and  57,  A).  The  cnido- 
cil  when  touched  was 
for  a  long  time  sup- 
posed to  cause  the 
explosion  of  the  nema- 
tocysts,  and  for  this 
reason  is  known  as  a 
"  trigger."  One  can 
easily  prove,  however, 
that  mechanical  shocks 
have  no  influence  upon 
the  nematocysts.  In- 
ternal pressure  produced 
either  by  distortion  or 
by  osmosis,  is  effective. 
For  this  reason  chemi- 
cals which  increase  the 
osmotic  pressure  within 
the  cnidoblast  cause  the 
eversion  of  the  thread- 
like tube  (123,  1 08). 
An  animal  when  "  shot" 
by  nematocysts  is  im- 
mediately paralyzed, 
and  sometimes  killed, 
by  a  poison  called  hyp- 
notoxin  which  is  in- 
jected into  it  by  the  FlG'  56-  Nematocysts  before  and  after  dis- 
,  i  charge.  I,  threadlike  tube  ;  new.,  nemato- 

'  cyst ;   cnc.,  cnidocil ;    »??/.,  nucleus  of  cni- 

Nematocysts   are  de-  doblast.     (From    Dahlgren    and    Kepner 

veloped  from  interstitial         after  Schneider.) 


nu. 


/ 


122 


AN  INTRODUCTION  TO  ZOOLOGY 


cells,  each  cell  producing  one  nematocyst.  "  First  a  clear  space  ap 
pears  in  an  interstitial  cell ;  this  space  enlarges,  it  acquires  a  defi- 
nite wall,  and  its  contents  stain  deeply.  Presently  it  elongates,  and 
one  end  is  produced  to  form  the  thread,  which  at  its  first  appear- 
ance is  everted  and  coiled  round  the  outside  of  the  sac.  After  a 
time  the  thread  is  introverted  —  it  is  not  quite  clear  how  —  and 


x:::::--0 


FIG.  57.  The  action  of  nematocysts.  A,  a  nematocyst  piercing  the  chitinous 
covering  of  an  insect ;  B,  nematocysts  holding  a  small  animal  by  coiling 
about  its  spines.  (After  Toppe  in  Zool.  Anz.} 


HYDRA  AND   CCELENTERATES  IN  GENERAL          123 

tne  nematocyst  assumes  its  final  form.  When  nearly  ripe  a  nema- 
tocyst,  still  contained  in  its  mother  cell  or  cnidoblast,  migrates  into 
the  inside  of  an  epitheliomuscular  cell  and  approaches  the  surface. 
The  external  end  of  the  cnidoblast  is  produced  to  form  a  cnidocil 
which  perforates  the  cuticle.  .  .  ."  (101,  p.  259.)  Since  the 
tube  of  the  nematocyst  cannot  be  returned  to  the  capsule,  nor 
another  one  be  developed  by  the  cnidoblast,  new  capsules  must 
be  formed  from  interstitial  ceils  to  replace  those  already  exploded. 

The  second  kind  of  nematocyst  is  cylindrical  (Fig.  55,  A)  and 
contains  a  thread  which  lacks  the  barbs  so  characteristic  of  its 
larger  neighbor.  The  third  variety  is  almost  spherical  and  smaller 
than  the  others,  measuring  only  .005  mm.  in  diameter.  The 
thread  contained  in  this  nematocyst  likewise  bears  no  barbs,  but 
when  discharged  resembles  a  corkscrew  (Fig.  57  B).  It  aids  in 
the  capture  of  prey  by  coiling  around  the  spines  or  other  struc- 
tures that  may  be  present  (121). 

The  interstitial  cells  also  develop  at  a  certain  period  of  the  year 
(September  and  October)  into  germ  cells.  The  origin  and  his- 
tory of  these  cells  will  be  found  fully  described  on  pages  134-136. 

The  BASAL  DISK  (54,  b.  d)  differs  somewhat  in  function  from  the 
rest  of  the  body.  It  is  the  point  by  which  Hydra  attaches  itself 
to  solid  objects,  and  for  this  purpose  secretes  a  sticky  substance. 
It  is  also  said  to  effect  the  movement  of  the  animal  from  place  to 
place  by  a  sort  of  gliding  motion,  not  yet  fully  explained,  but 
possibly  brought  about  by  pseudopodia-like  processes  thrust  out 
from  some  of  the  cells.  Epitheliomuscular  cells  and  a  few  inter- 
stitial cells  are  present,  but  no  nematocysts  are  to  be  found  here. 
The  columnar  epitheliomuscular  cells  are  not  only  provided  with 
contractile  fibers  at  their  bases,  but,  being  secretory,  also  contain 
a  large  number  of  small  refringent  granules,  as  shown  in  Figure  58, 
ad.  sez. 

The  TENTACLES  (Fig.  54,  /)  are  provided  with  an  ectoderm 
consisting  of  large  flat  cells,  thin  at  the  edges  and  thick  in  the 
center.  The  thicker  portions  give  the  surface  of  the  tentacle 
a  lumpy  appearance.  In  the  center  of  each  thickening  is  a 


124  AN  INTRODUCTION  TO  ZOOLOGY 

nucleus  around  which  are  embedded  sometimes  as  many  as  twelve 
nematocysts  each  in  its  own  cnidoblast  (Fig.  55,  A).  The  cni- 
docils  projecting  from  the  cnidoblasts  resemble  groups  of  cilia. 
Each  cnidoblast  is  drawn  out  at  its  base  into  a  contractile  fibril 
which  enters  the  longitudinal  muscular  sheet  at  the  base  of  the 
ectoderm  cells. 

ENTODERM  (Fig.  54,  en.).  — The  inner  layer  of  cells,  the  ento- 
derm,  occupies  about  two  thirds  of  the  body  wall.  Its  functions 

are  digestive  and  secretory.     The  di- 
gestive  cells   are   long   and   club- 
shaped,  with   transverse   muscular 
fibrils  at  their  base,  forming  a  cir- 
cular sheet  of  contractile  substance. 
At  the  larger  end,  which   extends 
-nto     ^     central     gastro  vascular 
FIG.  58.  Three   glandular    cells  cavity,  are   two  flagella.     Pseudo- 
horn  the  basal  disk  of  Hy-  p0dia  may  also  be  thrust  out  from 
dra       ad.  sec.,  _  granules    of  this  free  en(L      The  internal  struc- 
adhesive   secretion.      (From  A  r   ,  „     ,.~       ,     . 

Dahlgren  and  Kepner.)  ture  °f  these  Cells  dlffers  before  and 

after  the  animal  is  fed.  In  a  starv- 
ing Hydra  large  vacuoles  appear,  almost  completely  filling  the 
cell,  the  protoplasm  being  reduced  to  a  thin  layer  near  the  cell 
wall ;  after  a  meal,  however,  the  cells  are  gorged  with  nutritive 
spheres,  many  of  which,  especially  the  oil  globules,  migrate  into 
the  ectoderm  and  are  stored  near  the  periphery,  giving  the 
animal  its  brown  color  (104). 

The  glandular  cells  are  smaller  than  the  digestive  cells,  and  lack 
the  contractile  fibrils  at  their  base.  They  are  broad  at  the  free 
end,  and  thin  out  to  a  fine  filament  which  ends  in  a  knoblike 
enlargement  when  the  mesoglea  is  reached.  The  gland  cells 
also  differ  in  appearance  according  to  their  metabolic  activity: 
some  are  filled  with  large  vacuoles  containing  secretory  matter, 
while  others,  having  discharged  their  secretum,  appear  crowded 
with  fine  granules.  Interstitial  cells  are  found  lying  at  the  base 
of  the  other  entoderm  cells. 


HYDRA  AND   CCELENTERATES  IN   GENERAL          125 

The  tentacle  contains  entoderm  cells  apparently  devoid  of 
muscular  fibers.  Gland  cells  are  also  absent  from  this  region. 
The  entoderm  of  the  basal  disk  is  provided  with  only  a  few  glan- 
dular cells. 

MESOGLEA  (Fig.  54,  mes.).  —  The  mesoglea  in  Hydra  is  so  thin 
as  to  be  difficult  to  find,  even  when  highly  magnified;  in  some  of 
the  other  Ccelenterates  this  layer  is  very  thick,  constituting  by 
far  the  largest  part  of  the  body. 

NERVOUS  SYSTEM.  —  From  recent  investigations  it  seems  well 
established  that  Hydra  possesses  a  nervous  system,  though  com- 
plicated staining  methods  are  necessary  to  make  it  visible.  In 
the  ectoderm  there  is  a  sort  of  plexus  of  nerve  cells  connected  by 
nerve  fibers  with  centers  in  the  region  of  the  mouth  and  foot. 
Sensory  cells  in  the  surface  layer  of  cells  serve  as  external  organs 
of  stimulation,  and  are  in  direct  continuity  with  fibers  from  the 
nerve  cells.  Some  of  the  nerve  cells  send  processes  to  the  muscle 
fibers  of  the  epitheliomuscular  cells,  and  are  therefore  motor  in 
function.  No  processes  from  the  nerve  cells  to  the  nematocysts 
have  yet  been  discovered,  though  they  probably  occur.  The 
entoderm  of  the  body  also  contains  nerve  cells,  but  not  so  many 
as  are  present  in  the  ectoderm  (109). 

Nutrition.  —  FOOD.  —  The  food  of  Hydra  consists  principally 
of  small  animals  that  live  in  the  water.  Of  these  may  be  men- 
tioned small  Crustacea  such  as  Cyclops,  Annelids,  and  insect 
larvae.  Hydra  normally  rests  with  its  basal  disk  attached  to  some 
object  and  its  body  and  tentacles  extended  out  into  the  water. 
In  this  position  it  occupies  a  considerable  amount  of  hunting 
territory.  Any  small  aquatic  animal  swimming  in  touch  with  a 
tentacle  is  at  once  shot  full  of  nematocysts  (Fig.  55,  B),  which  not 
only  seem  to  paralyze  it,  but  also  to  hold  it  firmly.  There  is 
some  evidence  to  prove  that  the  tentacles  are  able  to  secrete  a 
fluid  which  serves  to  paralyze  the  animal  without  the  aid  of 
nematocysts  (123).  The  viscid  surface  of  the  tentacle  aids  in 
making  sure  that  the  victim  does  not  escape. 

INGESTION.  —  Ingestion    takes  place   as    follows:  First,  the 


126  AN  INTRODUCTION  TO  ZOOLOGY 

tentacle,  which  has  captured  the  prey,  bends  toward  the  mouth 
with  its  load  of  food.  The  other  tentacles  not  only  assist  in  this, 
but  may  use  their  nematocysts  in  quieting  the  victim.  The 
mouth  often  begins  to  open  before  the  food  has  reached  it.  The 
edges  of  the  mouth  gradually  inclose  the  organism  and  force  it 
into  the  gastrovascular  cavity.  The  body  wall  contracts  behind 
the  food  and  forces  it  down  until  it  reaches  the  basal  end  of  the 
body.  Here  it  remains  during  the  process  of  digestion.  Fre- 
quently organisms  many  times  the  size  of  the  Hydra  are  success- 
fully ingested. 

REACTIONS  TO  FOOD.  — It  is  not  uncommon  to  find  Hydras  that 
will  not  react  to  food  when  it  is  presented  to  them.  This  is  due 
to  the  fact  that  these  animals  will  eat  only  when  a  certain  interval 
of  time  has  elapsed  since  their  last  meal.  The  physiological 
condition  of  Hydra,  therefore,  determines  its  response  to  the  food 
stimulus.  The  collision  of  an  aquatic  organism  with  the  tentacle 
of  Hydra  is  not  sufficient  to  cause  the  food-taking  reaction,  since 
it  has  been  found  that  not  only  a  mechanical  stimulus,  but  also  a 
chemical  stimulus  must  be  present.  A  very  hungry  Hydra  will 
even  go  through  the  characteristic  movements  when  it  is  excited 
by  the  chemical  stimulus  alone.  This  has  been  shown  by  the 
following  experiment.  When  the  tentacles  and  hypostome  of  a 
moderately  hungry  Hydra  are  brought  into  contact  with  a  piece 
of  filter  paper,  which  has  been  soaked  for  a  time  in  the  same  cul- 
ture medium,  there  is  no  response.  If  the  filter  paper  is  then 
soaked  in  beef  juice  and  offered  to  the  Hydra,  the  usual  food 
reactions  are  given. 

Beef  juice  alone  calls  forth  no  response  in  a  moderately  hungry 
animal;  but  does  inaugurate  the  normal  reflex,  if  a  very  hungry 
specimen  is  selected  for  the  experiment.  The  conclusion  reached 
is  that  well-fed  Hydras  will  not  respond  to  either  mechanical  or 
chemical  stimuli  when  acting  alone  or  in  combination;  that 
moderately  hungry  animals  will  react  to  a  combination  of  the 
two,  and  that  hungry  animals  will  exhibit  food-taking  movements 
even  if  a  chemical  stimulus  alone  is  employed  (123). 


HYDRA  AND   CCELENTERATES  IN  GENERAL          127 

DIGESTION. — Immediately  after  the  ingestion  of  food  the  gland 
cells  in  the  entoderm  show  signs  of  great  activity;  their  nuclei 
enlarge  and  become  granular.  This  is  due  probably  to  the  forma- 
tion of  enzymes  which  are  discharged  into  the  gastrovascular 
cavity  and  begin  at  once  the  dissolution  of  the  food.  The  action 
of  the  digestive  juices  is  made  more  effective  by  the  churning  of 
the  food  as  the  animal  expands  and  contracts.  The  cilia  extend- 
ing out  into  the  central  cavity  also  aid  in  the  dissolution  of  the 
food  by  creating  currents.  This  method  of  digestion  differs  from 
that  of  Ameba  and  Paramecium  in  being  carried  on  outside  of  the 
cell;  i.e.  extracellular.  There  is  evidence  that  intracellular 
digestion  also  takes  place  in  Hydra;  the  pseudopodia  thrust  out 
by  the  entoderm  cells  seize  and  engulf  particles  of  food  which  are 
dissolved  within  the  cells.  However,  most  of  the  food  is  di- 
gested in  the  gastrovascular  cavity.  The  digested  food  is 
absorbed  by  the  entoderm  cells;  part  of  it,  especially  the  oil 
globules,  is  passed  over  to  the  ectoderm,  where  it  is  stored. 

EGESTION.  —  All  insoluble  material  is  egested  from  the  mouth. 
This  is  accomplished  by  "  a  very  sudden  squirt  "  which  throws 
the  debris  to  some  distance  (123). 

Behavior.  —  Hydra  mridis  gives  a  more  prompt  and  decisive 
response  when  stimulated  than  any  other  species  of  Hydra,  and 
for  this  reason  its  behavior  has  be  n  studied  more  thoroughly 
than  that  of  the  others.  The  following  paragraphs  have  been 
compiled  largely  from  experiments  upon  green  Hydras,  although 
enough  work  has  been  done  with  other  forms  to  prove  that  their 
reactions  are  practically  the  same,  only  more  sluggish. 

NORMAL  POSITION  OF  HYDRA.  — Hydras  maybe  found  attached 
to  the  sides  or  bottom  of  an  aquarium,  to  parts  of  water  plants, 
or  hanging  from  the  surface  film.  Usually  they  are  near  the  top 
where  more  oxygen  can  be  obtained  from  the  water  than  at  greater 
depths.  If  attached  to  the  bottom,  the  body  is  usually  held 
upright;  if  to  the  sides,  the  body  is  in  most  cases  horizontal, 
the  hypostome  generally  being  lower  than  the  foot;  and  if  to 
the  surface  film,  the  body  is  allowed  to  hang  directly  downward. 


128  AN  INTRODUCTION  TO  ZOOLOGY 

Suspension  from  the  surface  film  may  be  compared  with  that  of  a 
needle  placed  on  the  surface  of  the  water  (122).  Threads  of  a 
gelatinous  substance,  extending  out  from  the  basal  disk,  help 
sustain  the  body,  while  in  some  cases  a  large  air  bubble  attached 
to  the  foot  keeps  the  animal  afloat  (123).  The  position  of  rest 
in  every  case  gives  the  Hydra  the  greatest  opportunities  for  cap- 
turing food,  since  in  this  condition  it  has  control  of  a  large  amount 
of  territory. 


FIG.  59.  Spontaneous  changes  of  positions  in  an  undisturbed  Hydra.  Side 
view.  The  extended  animal  (i),  contracts  (2),  bends  to  a  new  position 
(3),  and  then  extends  (4).  (From  Jennings.) 

SPONTANEOUS  MOVEMENTS.  —  All  the  movements  of  Hydra 
are  the  result  of  the  expansion  or  contraction  of  the  muscle  fibers, 
and  are  produced  by  two  kinds  of  stimuli,  internal,  or  spontane- 
ous, and  external.  Spontaneous  movements  may  be  observed 
when  the  animal  is  attached  and  undisturbed.  At  intervals  of 
several  minutes  the  body,  or  tentacles,  or  both,  contract  suddenly 
and  rapidly,  and  then  slowly  expand  in  a  new  direction.  Hungry 
specimens  are  more  active  than  well-fed  individuals.  The  result 


HYDRA  AND   CCELENTERATES   IN   GENERAL 


I2Q 


is  to  bring  the  animal  into  a  new  part  of  its  surroundings,  where 
more  food  may  be  present  (Fig.  59).  These  movements  finally 
cease,  and  the  animal's  position  is  changed  by  locomotion. 


FIG.  60.  Hydra  moving   like   a   measuring   worm.     (From  Jennings  aftei 

Wagner.) 

LOCOMOTION.  —  Movement  from  place  to  place  is  effected  in 
one  of  three  ways.  In  most  cases  the  animal  bends  over  (Fig. 
60,  i)  and  attaches  itself  to  the  substratum  by  its  tentacles 
(2),  probably  with  the  aid  of  pseudopodia  thrust  out  by  the  ecto- 
derm cells.  The  basal  disk  is  then  released  and  the  animal  con- 


130 


AN  INTRODUCTION  TO  ZOOLOGY 


tracts  (Fig.  60,  3).  It  then  expands  (4),  bends  over  in  some  other 
direction  and  attaches  its  foot  (5).  The  tentacles  now  loosen 
their  hold  and  an  upright  position  is  regained  (6).  The  whole 
process  has  been  likened  to  the  looping  locomotion  of  a  measur- 
ing worm.  At  other  times  the  animal  moves  from  place  to  place 
while  inverted  by  using  its  tentacles  as  legs.  Locomotion  may 
also  result  from  the  gliding  of  the  foot  along  the  substratum,  and 
considerable  distances  are  sometimes  covered  in  this  way. 

REACTIONS  TO  EXTERNAL  STIMULI.  —  THIGMOTROPISM. — 
Hydra  reacts  to  various  kinds  of  special  stimuli.  Reaction  to  con- 
tact accounts  for  its  temporary  fixed  condition.  The  attachment 
while  in  the  resting  attitude  is  a  result  of  this  reaction,  and  not  a 
response  to  gravity,  since  Hydras  have  the  longitudinal  axis  of  the 
body  directed  at  every  possible  angle  regardless  of  the  force  of 
gravity.  Mechanical  shocks,  such  as  the  jarring  of  the  watch 
glass  containing  a  specimen,  or  the  agitation  of  the  surface  of 
the  water,  cause  a  rapid  contraction  of  a  part  or  all  of  the  animal. 
This  is  followed  by  a  gradual  expansion  until  the  original  condi- 
tion is  regained. 

Mechanical  stimuli  may  be  localized  or  non-localized.  That 
just  noted  is  of  the  latter  type.  Local  stimulation  may  be  accom- 
plished by  touching  the  body  or  tentacles  with  the  end  of  a  fine 
glass  rod.  The  reactions  to  local  stimuli  variously  applied  have 
been  classified  as  follows :  — 

"  A.   Stimulation  of  body: 

1.  Weak:  body  partly  contracts. 

2.  Medium:  body  completely  contracts. 

3.  Strong:  body  and  tentacles  contract. 
B.   Stimulation  of  a  tentacle: 

1.  Weak:  tentacle  stimulated  contracts. 

2.  Medium:  all  tentacles  contract. 

3.  Strong:  tentacles  and  body  contract  "  (123,  p.  594). 

It  should  be  noted  that  the  stimulation  of  one  tentacle  may  cause 
the  contraction  of  all  the  tentacles  (B,  2),  or  even  the  contrac- 


HYDRA  AND   COELENTERATES  IN   GENERAL          131 

tion  of  both  tentacles  and  body  (B,  3).  This  shows  that  there 
must  be  some  sort  of  transmission  of  stimuli  from  one  tentacle 
to  another  and  to  the  body.  The  structure  of  the  nervous  sys- 
tem would  make  this  possible  (see  p.  125) 

PHOTOTROPISM.  —  There  is  no  definite  response  to  light,  al- 
though the  final  result  is  quite  decisive.  If  a  dish  containing  Hy- 
dras is  placed  so  that  the  illumination  is  unequal  on  different  sides, 
the  animals  will  collect  in  the  brightest  region,  unless  the  light 
is  too  strong,  in  which  case  they  will  congregate  in  a  place  where 
the  light  is  less  intense.  Hydra  therefore  has  an  optimum  with 
regard  to  light.  The  movement  into  or  out  of  a  certain  area  is 
accomplished  by  a  method  of  "  trial  and  error."  When  put 
in  a  dark  place  Hydra  becomes  restless  and  moves  about  in  no 
definite  direction;  but  if  white  light  is  encountered,  its  locomotion 
becomes  less  rapid  and  finally  ceases  altogether.  The  value  to 
the  organism  of  such  a  reaction  is  quite  important,  since  the  small 
animals  that  serve  as  food  for  it  are  attracted  to  well-lighted  areas. 
Colored  lights  have  the  same  effect  as  darkness;  blue,  however, 
is  preferred  by  Hydra  to  white. 

THERMOTROPISM.  —  The  reactions  of  Hydra  to  changes  in 
temperature  are  also  indefinite,  although  in  many  cases  they 
enable  the  animal  to  escape  from  a  heated  region.  No  loco- 
motory  change  is  produced  by  temperatures  below  31°  C.;  at 
this  temperature,  however,  the  basal  disk  is  released  and  the 
animal  takes  up  a  new  position  either  away  from  the  heated  area 
or  further  into  it.  In  the  former  case  the  Hydra  escapes,  in  the 
latter  it  may  escape  if  subsequent  movements  take  it  away  from 
the  injurious  heat,  otherwise  it  perishes.  Hydra  does  not  move 
from  place  to  place  if  the  temperature  is  lowered;  it  contracts 
less  rapidly,  and  finally  ceases  all  its  movements  when  the  freez- 
ing point  is  approached  (113). 

,  ELECTROTROPISM.  —  An  attached  Hydra,  when  subjected  to  a 
weak  constant  electric  current,  bends  toward  the  anode,  its  body 
finally  becoming  oriented  with  the  basal  disk  toward  the  cathode 
and  the  anterior  end  toward  the  anode  side.  The  entire  animal 


132  AN  INTRODUCTION  TO  ZOOLOGY 

then  contracts.  In  an  animal  attached  by  the  tentacles  a  similai 
bending  occurs,  but  the  basal  disk  in  this  case  is  directed  toward 
the  anode.  These  reactions  are  caused  by  local  contractions  on 
the  anode  side  for  which  the  electric  current  is  directly  respon- 
sible (115). 

Hydra  shows  no  rheotropic  reactions.  When  placed  in  a  cur- 
rent of  water  it  neither  orients  itself  in  a  definite  way  nor  moves 
either  up  or  down  stream  (123). 

GENERAL  REMARKS  ON  THE  BEHAVIOR  OF  HYDRA.  —  It  is  evi- 
dent from  the  above  outline  of  the  reactions  of  Hydra  to  stimuli 
that  the  only  movements  involved  are  produced  by  contraction 
and  expansion  of  the  body  when  attached,  and  by  undirected 
changes  of  position.  Being  radially  symmetrical,  the  body  may 
be  flexed  in  any  direction. 

Local  stimuli,  such  as  the  application  of  heat  or  a  chemical  to  a 
limited  area  of  the  body,  causes  a  contraction  of  the  part  affected 
and  a  bending  in  that  direction.  This  results  in  the  movement 
of  the  tentacular  region  toward  the  stimuli,  and  the  contraction 
of  the  entire  animal  follows,  thus  carrying  it  out  of  the  influence 
of  the  stimulus. 

Non-localized  stimuli,  such  as  the  jarring  of  the  vessel  contain- 
ing the  animals,  produces,  immediately,  a  contraction  of  the 
entire  body,  which,  in  most  cases,  is  beneficial,  since  it  removes  it 
from  an  injurious  agent.  If,  however,  this  simple  contraction 
is  not  effective,  as  in  the  case  of  a  constant  application  of  heat, 
the  Hydra  usually  resorts  to  some  other  reaction,  e.g.  locomotion, 
which  often  enables  it  to  escape  from  the  injurious  stimulus. 

Finally,  it  should  be  remembered  that  the  physiological  condi- 
tion of  the  animal  determines  to  a  large  extent  the  kind  of  reac- 
tions produced,  not  only  spontaneously,  but  also  by  external 
stimuli.  "  It  decides  whether  Hydra  shall  creep  upward  to  the 
surface  and  toward  the  light,  or  shall  sink  to  the  bottom;  how  it 
shall  react  to  chemicals  and  to  solid  objects;  whether  it  shall 
remain  quiet  in  a  certain  position,  or  shall  reverse  this  position 
and  undertake  a  laborious  tour  of  exploration  "  (no,  p.  231). 


HYDRA  AND   CCELENTERATES  IN   GENERAL          133 

Reproduction.  —  Reproduction  takes  place  in  Hydra  both 
asexually  and  sexually;  in  the  former  case,  by  fission  and  budding, 
in  the  latter,  by  the  production  of  a  fertilized  egg. 

LONGITUDINAL  FISSION.  —  Since  the  work  of  Trembley  (1744) 
appeared,  a  number  of  zoologists  have  reported  the  discovery  of 
double  Hydras.  These  were  considered  by  some  as  abnormalities, 
and  by  others  as  undergoing  the  process  of  longitudinal  fission. 
There  seems  now  to  be  plenty  of  evidence  to  prove  that  Hydra 
does  reproduce  by  longitudinal  division  (112).  The  distal  end 
of  the  animal  divides  first;  then  the  body  slowly  splits  down  the 
center,  the  halves  finally  separating  when  the  basal  disk  is  severed 


FIG.  61.  Hydra  reproducing  by  longitudinal  division. 
(After  Koelitz  in  Zool.  Anz.) 

(Fig.  61).  Hydras  hsive  also  been  found  which  bore  buds  repro- 
ducing in  this  manner.  This  method  of  multiplication  must, 
however,  be  rare  since  it  is  so  seldom  seen.  Transverse  fission  has 
also  been  reported  (m). 

BUDDING  (Fig.  54,  b). — A  commoner  method  of  asexual 
reproduction,  and  one  that  is  easily  observed  in  the  laboratory,  is 
by  budding.  Superficially  the  bud  appears  first  as  a  slight  bulge 
in  the  body  wall.  This  .pushes  out  rapidly  into  a  stalk  which 
soon  develops  a  circlet  of  blunt  tentacles  about  its  distal  end. 
The  cavities  of  both  stalk  and  tentacles  are  at  all  times  directly 


134  AN  INTRODUCTION  TO  ZOOLOGY 

connected  with  that  of  the  parent.  When  full  grown,  the  bud 
becomes  detached  and  leads  a  separate  existence.  The  details 
of  the  process  are  briefly  as  follows.  The  interstitial  cells  in  a 
certain  region  increase  in  number  and  volume,  producing  a  slight 
outbulging  of  the  ectoderm.  The  growing  region  is  located  at 
the  point  where  the  edges  of  the  protrusion  meet  the  body  wall. 
Here  the  cells  are  well  fed  and  multiply  actively.  The  ectoderm 
and  entoderm  cells  of  the  parent  give  rise  to  the  corresponding 
cells  of  the  bud.  When  the  bud  is  fully  grown,  the  ectoderm  cells 
at  its  proximal  end  secrete  a  sticky  substance  which  is  used  later 
for  its  attachment.  The  entoderm  cells  in  the  same  region  then 
unite,  separating  the  cavity  of  the  bud  from  that  of  the  parent. 
Finally,  the  bud  becomes  detached.  The  food  supply  determines 
the  rate  of  growth  of  the  bud,  and  a  bud  may  be  entirely  absorbed 
by  a  starving  animal  (120). 

SEXUAL  REPRODUCTION.  —  Whether  or  not  there  are  definite 
germ  cells  in  the  adult  Hydra  is  still  open  to  question.  So  far 
as  is  known,  both  ova  and  spermatozoa  arise  from  indifferent 
interstitial  cells. 

SPERMATOGENESIS.  —  The  male  cells  of  Hydra  are  formed  in 
little  conical  elevations  called  testes  which  project  from  the  surface 
of  the  body  (Fig.  54,  y.  t.,m.t.).  The  testis  arises  within  the  ecto- 
derm from  interstitial  cells.  A  single  interstitial  cell  divides 
mitotically;  then  adjacent  interstitial  cells  also  divide,  multi- 
plication continuing  until  the  ectoderm  becomes  distended.  An 
indefinite  number  of  long  multinucleated  cysts  (Fig.  62,  A)  are 
formed  within  the  testis,  each  cyst  being  the  product  of  a  single 
or  several  interstitial  cells.  Each  interstitial  cell  is  a  primordial 
germ  cell ;  it  gives  rise  by  mitosis  to  a  variable  number  of  sper- 
matogonia,  which  contain  the  somatic  number  of  chromosomes, 
twelve.  Reduction  in  the  number  of  chromosomes  to  six  occurs 
just  after  the  spermatogonia  have  divided  to  form  the  primary 
spermatocytes  (Fig.  62 ,  A,  b).  The  latter  give  rise  to  secondary 
spermatocytes  (c)  which  divide  at  once,  producing  spermatids 
(c).  These  two  spermatocyte  divisions  take  place  without  the 


HYDRA  AND   CCELENTERATES  IN   GENERAL 


formation  of  cell  walls,  i.e.  each  primary  spermatocyie  develops 
into  a  four-nucleated  cell  which  represents  the  four  spermatids. 
Within  this  cell  the  spermatids  transform  into  spermatozoa 
(Fig.  62,  B).  A  single  cyst  may  contain 
representatives  of  all  of  these  cell  gener- 
ations—  spermatogonia,  primary  sper- 
matocytes, secondary  spermatocytes, 
spermatids,  and  spermatozoa.  The 
mature  spermatozoa  break  out  of  the 
vesicle  in  which  they  are  formed,  and 
swim  about  in  the  distal  end  of  the  cyst 
(Fig.  62,  A,  d);  they  finally  reach  the 
outside  by  way  of  a  minute  temporary 
opening  in  the  end  of  the  cyst.  The 
mature  spermatozoa  swim  about  in  the 
water  searching  for  an  egg;  their  activ- 
ity continues  from  one  to  three  days. 

OOGENESIS.  —  The  egg  is  first  dis- 
tinguished from  the  interstitial  cells  of 
the  ectoderm  by  its  slightly  greater  size, 
its  spherical  shape,  and  the  compara- 
tively large  volume  of  its  nucleus.  As 
the  eggs  grow  in  size  the  neighboring 
interstitial  cells  increase  in  number  by 
mitosis,  and  also  become  larger.  The 
whole  structure  may  at  this  time  be 

called  an  ovary  (Figs.  63,  A;  54,  y.  e.).   FlG-  62'  Parts  of  a  testis  of 

Hydra.   A,  a  single  cyst 

showing      spermatogo- 
nia, primary  spermato- 


The  nourishment  of  the  egg  is  at  first 
similar  to  that  of  the  other  ectoderm 
cells,  but  later  the  interstitial  cells  near 
it  are  engulfed,  their  contents  becoming 
part  of  the  ovum.  The  nuclei  of  these 
interstitial  cells  furnish  the  yolk  of  the 
growing  egg.  Usually  only  one  egg  is 
developed  in  a  single  ovary,  but  some- 


cytes  (b),  secondary 
spermatocytes  and 
spermatids  (r),  and 
spermatozoa  (d) ;  B, 
developing  spermato- 
zoa. (After  Tann- 
reuther  in  Biol.  Bui.) 


AN  INTRODUCTION  TO  ZOOLOGY 


times  two  may  arise  and  complete  their  development  side  by  side. 
In  most  cases,  however,  when  two  or  more  eggs  are  contained  in 
one  ovary,  their  adjacent  walls  dissolve  and  one  of  the  nuclei  sur- 
vives while  the  others  disintegrate.  As  the  ovum  grows  it  becomes 


FIG.  63.  An  ovary  (A),  and  mature  egg  (B)  of 
Hydra,  a,  egg  nucleus ;  ps.,  pseudopodium 
of  young  egg  ;  em,  egg  membrane.  (After 
Tannreuther  in  Biol.  Bui.) 


ameboid  in  shape,  showing  distinct  pseudopodia  (Fig.  63,  A,  p.s.) ; 
these  are  drawn  in  when  it  has  reached  its  full  size.  The  egg  is 
now  nearly  spherical,  and  is  surrounded  by  a  single  layer  of  ecto- 
derm cells  (Fig.  54,  m.e.).  Maturation  then  takes  place.  Two 
polar  bodies  (p.b.)  are  formed,  the  first  being  larger  than  the  sec- 
ond. During  maturation  the  number  of  chromosomes  is  reduced 
from  the  somatic  number,  twelve,  to  six.  This  occurs  at  the  end 
of  the  growth  period.  Now  an  opening  appears  in  the  ectoderm 
and  the  egg  is  forced  out,  finally  becoming  free  on  all  sides  except 
where  attached  to  the  animal  (Fig.  63,  B). 


FIG.  64.  Regeneration  and  grafting  in  Hydra.  A,  seven-headed  Hydra  made 
by  splitting  distal  ends  lengthwise ;  B,  a  piece  of  Hydra  regenerating 
an  entire  animal ;  1-5,  stages  in  this  process ;  C,  part  of  one  Hydra 
grafted  upon  another.  (From  Morgan,  A  after  Trembley,  B  after 
Morgan,  C  after  King.) 


HYDRA  AND   CCELENTERATES  IN  GENERAL          137 

FERTILIZATION. —  Fertilization  usually  occurs  within  two  hours. 
Several  sperms  penetrate  the  egg  membrane,  but  only  one  enters 
the  egg  itself.  If  not  fertilized  within  twenty-four  hours,  the  egg 
becomes  sterile.  The  sperm  brings  a  nucleus  containing  six 
chromosomes  into  the  egg.  The  male  and  female  nuclei  unite, 
forming  the  fusion  nucleus. 

EMBRYOLOGY.  —  Cleavage,  which  now  begins,  is  total  and  regu- 
lar. A  well-defined  cleavage  cavity  is  present  at  the  end  of  the 
third  cleavage,  i.e.  the  eight-celled  stage.  When  the  blastula  is 
completed,  it  resembles  a  hollow  sphere  with  a  single  layer  of 
epithelial  cells  composing  its  wall.  These  cells  may  be  called  the 
primitive  ectoderm.  By  mitotic  division  they  form  entoderm 
cells  which  drop  into  the  cleavage  cavity,  completely  filling  it. 
The  gastrula,  therefore,  is  a  solid  sphere  of  cells  differentiated 
into  a  single  outer  layer,  the  ectoderm  and  an  irregular  central 
mass,  the  entoderm.  The  ectoderm  surrounds  the  gastrula  with 
two  envelopes.  The  outer  is  a  thick  chitinous  shell  covered 
with  sharp  projections;  the  inner  is  a  thin  gelatinous  membrane. 

HATCHING.  —  The  embryo  in  this  condition  separates  from  the 
parent  and  falls  to  the  bottom,  where  it  remains  unchanged  for 
several  weeks.  Then  interstitial  cells  make  their  appearance.  A 
subsequent  resting  period  is  followed  by  the  breaking  away  of 
the  outer  chitinous  envelope  and  the  elongation  of  the  escaped 
embryo.  Mesoglea  is  now  secreted  by  the  ectoderm  and  ento- 
derm cells;  a  circlet  of  tentacles  arises  at  one  end  and  a  mouth 
appears  in  their  midst.  The  young  Hydra  thus  formed  soon 
grows  into  the  adult  condition. 

Regeneration.  —  An  account  of  the  phenomenon  of  regenera- 
tion is  appropriate  at  this  place,  since  the  power  of  animals  to 
restore  lost  parts  was  first  discovered  in  Hydra  by  Trembley 
in  1740.  This  investigator  found  that  if  Hydras  were  cut  into 
two,  three,  or  four  pieces,  each  part  would  grow  into  an  entire 
animal.  Other  experimental  results  obtained  by  Trembley  are 
that  the  hypostome  together  with  the  tentacles,  if  cut  off,  produce 
a  new  individual;  that  each  piece  of  a  Hydra  split  longitudinally 


138  AN  INTRODUCTION  TO  ZOOLOGY 

into  two  or  four  parts,  becomes  a  perfect  polyp  (Fig.  64,  A) 
that  when  the  head  end  is  split  in  two  and  the  parts  separated 
slightly  a  two-headed  animal  results;  and  that  a  specimen  when 
turned  inside  out  is  able  to  readjust  itself  to  new  conditions  forced 
upon  it. 

Regeneration  may  be  denned  as  the  replacing  of  an  entire 
organism  by  a  part  of  the  same.  It  takes  place  not  only  in 
Hydra,  but  in  many  other  Ccelenterates,  and  in  some  of  the  rep- 
resentatives of  almost  every  phylum  of  the  animal  kingdom. 
Hydra,  however,  is  the  species  that  has  been  most  widely  used 
for  experimentation.  Pieces  of  Hydra  that  measure  J  mm.  or 
more  in  diameter  are  capable  of  becoming  entire  animals  (Fig. 
64,  B).  The  tissues  in  some  cases  restore  the  lost  parts  by  a 
multiplication  of  their  cells ;  in  other  cases,  they  are  worked  over 
directly  into  a  new  but  smaller  individual. 

GRAFTING.  —  Parts  of  one  Hydra  may  easily  be  grafted  upon 
another  (Fig.  64,  C).  In  this  way  many  bizarre  effects  have  been 
produced.  Parts  of  two  Hydras  of  two  species  have  also  been 
successfully  united. 

GENERAL  REMARKS  ON  REGENERATION.  —  Space  will  not  per- 
mit a  detailed  account  of  the  many  interesting  questions  involved 
in  the  phenomena  of  regeneration,  but  enough  has  been  given  to 
indicate  the  nature  of  the  process.  The  benefit  to  the  animal  of 
the  ability  to  regenerate  lost  parts  is  obvious  to  all.  Such  an 
animal,  in  many  cases,  will  succeed  in  the  struggle  for  existence 
under  adverse  conditions.  The  regeneration  of  the  earthworm 
and  the  crayfish  are  considered  in  Chapters  X  and  XI.  It  will 
suffice  here,  therefore,  to  say  a  few  words  concerning  regenera- 
tion in  general.  Regeneration  takes  place  continually  in  all 
animals;  for  example,  new  cells  are  produced  in  the  epidermis  of 
man  to  take  the  place  of  those  that  are  no  longer  able  to  perform 
their  proper  functions.  Both  internal  and  external  factors  have 
an  influence  upon  the  rate  of  regeneration  and  upon  the  character 
of  the  new  part.  Temperature,  food,  light,  gravity,  and  contact 
are  some  of  the  external  factors.  In  man,  various  tissues  are 


HYDRA  AND    CCELENTERATES  IN    GENERAL 

capable  of  regeneration;  for  example,  the  skin,  muscles,  nerves, 
blood  vessels,  and  bones.  Lost  parts  are  not  restored  in  man, 
because  the  growing  tissues  do  not  coordinate  properly.  Many 
theories  have  been  advanced  to  explain  regenerative  processes, 
but  none  has  gained  sufficient  acceptance  to  warrant  its  inclusion 
here. 

2.  CCELENTERATES  IN  GENERAL 

The   Characteristics   and   Classification   of    Ccelenterates.  - 

The  Phylum  Ccelenterata  includes  the  polyps,  jellyfishes,  sea 
anemones,  and  corals.  All  of  these  animals  have  a  body  wall 
consisting  of  two  layers  of  cells,  between  which  is  a  non-cellular 
substance,  the  mesoglea.  Within  the  body  is  a  single  gastro- 
vascular  cavity,  or  ccelenteron.  Because  of  the  presence  of  two 
cellular  layers,  all  Ccelenterates  are  said  to  be  diploblastic.  They 
are  also  acoelomates,  i.e.  they  do  not  possess  a  second  body  cavity, 
the  ccelom.  All  Ccelenterates  are  provided  with  nematocysts. 

This  phylum  contains  three  classes,  as  follows :  — 

Class  i,  Hydrozoa.  This  class  includes  the  fresh-water  polyps, 
the  small  jellyfishes,  the  hydroid  zoophytes,  and  a  few  stony 
corals. 

Class  2,  Scyphozoa.  Most  of  the  large  jellyfishes  are  placed 
in  this  class. 

.  Class  3,  Anthozoa.  In  this  class  are  found  the  sea  anemones, 
and  most  of  the  stony  and  horny  corals. 

Hydra  is  a  member  of  the  class  Hydrozoa.  It  may  be  considered 
as  a  type  of  what  is  known  as  a  zooid,  or  polyp.  Many  of  the 
Hydrozoa  living  in  salt  water  are  colonial,  consisting  of  a  large 
number  of  zooids  which  are  united  so  as  to  resemble  a  branching 
tree,  e.g.  Obelia  (Fig.  65,  A).  If  the  buds  of  a  Hydra  should 
remain  attached  to  their  parent,  and  should  in  turn  produce  buds 
a  hydroid  colony  somewhat  like  Obelia  would  result.  All  Hydro- 
zoa are  not  fixed,  but  some  of  them  swim  about  freely  through  the 
water.  The  jellyfishes,  or  medusce  (Fig.  65,  B),  are  of  this  type. 
They  are  cup-shaped  animals  with  a  circlet  of  tentacles  extending 


140 


AN  INTRODUCTION   TO  ZOOLOGY 


FIG.  65.  Part  of  .a  colonial  Hydrozoan,  Obelia  (A),  and  a  free-swimming 
medusa  (B)  from  another  hydroid  colony  (Bougainmllia).  i,  ectoderm  ; 
2,  entoderm ;  3,  mouth ;  4,  coelenteron ;  5,  coenosarc ;  6,  perisarc ; 
7,  hydrotheca  ;  8,  blastostyle ;  9,  medusa  bud ;  10,  gonotheca.  (From 
Shipley  and  MacBride.) 


HYDRA  AND   CCELENTERATES  IN   GENERAL 


141 


from  the  extreme  rim.     The  middle  layer,  the  mesoglea,  is  in 
them  remarkably  thick,  and  resembles  jelly,  hence  their  name. 

Hydra  and  Jellyfishes  Compared. — Although  the  medusie 
upon  superficial  examination  appear  to  be  very  different  from  the 
polyps  or  zooids,  they  are  constructed  on  the  same  general 


tcl 


FIG.  66.  Diagrams  showing  the  similarities  of  a  polyp  (A)  and  a  medusa  (B). 
circ.,  circular  canal ;  cc/.,  ectoderm  ;  end.,  entoderm  ;  cnt.'cav.,  gastro- 
vascular  cavity  ;  hyp.,  hypostome  ;  mnb.,  manubrium  ;  msgl.,  mesoglea  ; 
mth.,  mouth  ;  nv.,  nerve  rings ;  rod.,  radial  canal ;  v.,  velum.  (From 
Parker  and  Haswell.) 

plan  as  the  latter.  Figure  66  illustrates  in  a  diagrammatic 
fashion  the  resemblances  between  the  polyp  (A),  and  the  medusa 
(B)  by  means  of  longitudinal  sections.  If  the  medusa  were 
grasped  at  the  center  of  the  aboral  surface  and  elongated,  a  hydra- 
like  form  would  result.  Both  have  similar  parts,  the  most  no- 
ticeable difference  being  the  enormous  quantity  of  mesoglea 
present  in  the  medusa. 

Metagenesis.  —  In  some  Hydrozoa  there  are  two  kinds  of 
individuals  belonging  to  the  same  species;  one  of  these,  in  the 
form  of  a  polyp,  gives  rise  asexually  by  budding  to  the  second 
form,  the  medusa,  which  produces  eggs  and  sperms.  The  fer- 
tilized egg  develops  into  the  polyp.  The  polyp,  or  hydroid  stage, 


142  AN  INTRODUCTION  TO  ZOOLOGY 

is  more  pronounced  in  some  species  than  in  others,  e.g.,  Hydra 
has  no  medusa  stage  at  all;  whereas  certain  species  have  no 
polyp  stage  but  pass  their  entire  existence  as  medusae.  Various 
conditions  may  be  illustrated  by  different  Hydrozoa.  In  the 
following  table,  O  represents  the  fertilized  ovum;  H,  a  polyp; 
M,  a  medusa;  m,  an  inconspicuous  or  degenerate  medusa,  and 
h,  an  inconspicuous  or  degenerate  polyp  (103). 

1.  O  —  H  —  O  —  H  —  O  (Hydra). 

2.  O  —  H  —  m  —  O  —  H  —  m  —  O  (Sertularia). 

3.  O  —  H  —  M  —  O  —  H  —  M  —  0(0belia). 

4.  O  —  h  —  M  —  O  —  h  —  M  —  O  (Liriope). 

5.  O  —  M  —  O  —  M  —  O  (Geryonia). 

The  alternation  of  a  sexual  with  an  asexual  generation,  as  in 
examples  2,  3,  and  4,  just  listed,  is  known  as  metagenesis.  This 
phenomenon  occurs  in  other  groups  of  the  animal  kingdom,  but 
finds  its  best  examples  among  the  Ccelenterates. 

Division  of  Labor  among  Coelenterates.  —  Not  only  have  the 
somatic  cells  of  the  Coelenterates  become  differentiated  into  ecto- 
derm and  entoderm,  in  each  of  which  cells  may  be  recognized 
having  particular  functions  to  perform;  but  in  certain  groups 
colonial  species  are  found  in  which  the  various  members  of  the 
colony  are  so  specialized  for  certain  kinds  of  work,  that  they  are 
incapable  of  carrying  on  other  processes.  Perhaps  the  best 
example  of  such  a  condition  is  Physalia,  the  "  Portuguese  Man- 
of-War  "  (Fig.  67).  Physalia  is  a  colonial  Hydrozoan  consist- 
ing of  a  large  float  (pn.)  with  a  sail-like  crest  (cr.)  from  which  a 
number  of  polyps  hang  down  into  the  water.  Some  of  these 
polyps  are  nutritive,  others  are  tactile;  some  contain  batteries 
of  nematocysts,  others  are  male  reproductive  zooids,  and  still 
others  give  rise  to  egg-producing  medusae. 

Corals.  —  One  group  of  Ccelenterates,  the  corals,  is  of  especial 
interest  because  of  its  economic  importance.  The  corals  are 
found  principally  in  the  tropics.  They  live  near  the  shore,  which 
not  infrequently  is  built  up  of  coral  skeletons.  The  hard  parts 


cr* 


FIG.  68.  Association  of  a  hermit  crab  and  a 
Ccelenterate.  (From  Parker  and  Has- 
well.) 


FIG.  67.  Physalia,  the  "Portu- 
guese man-of-war."  cr., 
crest;  #.,  polyp;  />«., 
pneumatophore.  (From 
Parker  and  Haswell  after 
Huxley.) 


HYDRA  AND   CCELENTERATES  IN  GENERAL          143 

of  corals  are  composed  of  calcium  carbonate  excreted  by  the  ecto- 
derm cells  of  the  polyps.  So  numerous  are  these  polyps  that  many 
islands  in  the  Pacific  Ocean  and  many  reefs  near  other  islands  con- 
sist entirely  of  coral  rock.  The  precious  Red  Coral  is  found  only 
in  the  Mediterranean  Sea. 

Symbiosis.  —  Symbiosis  means  an  intimate  and  advantageous 
association  between  two  kinds  of  organisms.  The  most  common 
example  is  the  lichen,  which  consists  of  two  plants,  an  alga  and  a 
fungus.  Hydra  viridis,  the  green  Hydra,  derives  its  color  from  a 
great  number  of  unicellular  green  plants,  the  Zoochlorella,  which 
occupy  the  basal  portion  of  the  entoderm  cells.  These  green 
algae  manufacture  starch  in  the  presence  of  light,  and,  during 
this  process,  liberate  oxygen  which  is  of  advantage  to  the  polyp. 
Probably  the  Hydra  also  obtains  food  from  these  algae.  The 
security  and  carbon  dioxide  furnished  by  the  protecting  cells  of 
the  polyp  compensate  the  algae  for  the  food  and  oxygen  they 
provide. 

A  most  complicated  illustration  of  symbiosis  is  that  of  certain 
hermit  crabs  with  sea  anemones.  The  hermit  crab  lives  in  the 
shell  of  a  large  salt-water  snail.  As  soon  as  a  suitable  shell  is 
found,  the  hermit  crab  takes  possession.  It  then  hunts  about 
until  it  finds  a  sea  anemone,  which  it  places  upon  the  shell  just 
above  the  opening.  Often  the  anemone  completely  covers  the 
hermit  crab's  house.  The  advantage  to  the  sea  anemone  in  this 
partnership  lies  in  the  greater  chances  it  has  for  proper  food 
conditions,  since  it  is  carried  about  from  place  to  place  by  the 
crab.  The  benefits  to  the  latter  are  of  a  peculiar  character. 
Figure  68  shows  a  Ccelenterate  colony,  Hydractinia,  con- 
sisting of  nutritive  polyps  with  tentacles,  reproductive  individuals 
bearing  a  circle  of  medusoid  buds,  spine-like  protective  members, 
and  bordering  the  edge  of  the  shell,  a  row  of  threadlike  defensive 
polyps  provided  with  stinging  cells.  When  the  hermit  crab  is 
attacked,  these  stinging  cells  are  shot  into  the  enemy,  which  is 
thus  frequently  driven  away.  In  this  way  the  Ccelenterate  benefits 
its  associate. 


CHAPTER  IX 


SPONGES,   FLAT   WORMS,  AND   ROUND    WORMS  l 

i.  SPONGES  —  GRANTIA 
(Grantia  ciliata  Fiem.) 

Grantia  (Fig.  69)   is  a  simple  sponge   in- 
habiting the  salt  water  along  the  coast  of  the 
New  England  states  just  below  the  low-tide 
mark..    Here  it  is  found  attached  by  one  end 
to   rocks   and   other   solid    objects.     Unlike 
Hydra,    Grantia    is     permanently     attached, 
never  moving  from  place  to  place  as  an  adult. 
Its  distribution  in  space  is  effected  during 
the  early  embryonic  stages,  at  which  time 
cilia  are  present,  enabling  it  to  swim  about. 
Grantia  varies  in  length  from  one  half  an 
inch  to  almost    an    inch,  and  resembles  in 
shape  a   slender  vase   that   bulges  slightly 
FIG.  69.    A   simple    near    ^e    center.      The    distal   end    of   the 
sponge.     (After    animal  opens  to  the  exterior  by  a  large  ex- 
Minchin  in  Lan-    current  pore,  the  osculum.      This  opening  is 
surrounded  on  all  sides  by  a  circlet  of  long 
straight    needles    called    spicules.      Smaller 
spicules  protrude  from   other  parts   of   the  body,  giving    the 


kester's 
tise.) 


Trea- 


1  Since  it  was  impossible  to  include  in  this  book  detailed  discussions  of 
types  from  every  phylum  of  the  animal  kingdom,  many  groups  are  not  rep- 
resented. In  this  chapter  three  animals  are  briefly  described  in  order  that 
the  step  from  a  simple  Ccelenterate,  like  Hydra,  to  a  complex  Annelid,  like 
the  earthworm,  may  not  be  too  abrupt.  The  sponges,  flat  worms,  and 
round  worms  possess  certain  organs  that  help  the  student  to  understand  the 
structure  and  functions  of  similar  organs  in  more  complex  animals. 

144 


SPONGES,   FLAT  WORMS,  AND  ROUND   WORMS        145 

animal  a  ciliated  appearance.  The  body  wall  is  perforated  by 
numerous  incur  rent  pores.  This  characteristic  has  suggested  the 
name  Porifera  (Lat.  poms,  a  pore,  and  ferre,  to  bear)  to  mem- 
bers of  this  phylum. 

Structure.  —  A  specimen  of  Grantia  split  longitudinally  (Fig. 
70)  shows  the  body  to  be  a  hollow  sac,  one  large  central  cavity, 
the  cloaca,  being  pres- 
ent. The  body  wall  is 
honeycombed  by  a 
great  many  canals; 
some  of  these,  the 
radial  canals,  open  to 
the  cloacal  cavity 
through  minute  pores, 
the  apopyles,  and  end 
blindly  near  the  outer 
surface;  others,  the  in- 
current  canals,  open  to 
the  outside  through 
small  incurrent  pores 
or  ostia,  and  end 
blindly  near  the  inner 
surface  of  the  body 
wall;  still  other  canals, 
the  prosopyles,  even 
smaller  than  those  al- 
ready noted,  connect 
the  radial  with  the  in- 
current  canal.  Figure 


FIG.  70.    Longitudinal    section    of    a 

sponge,     ip.,  incurrent  pores;     o.,  oscu- 
lum.     (From  Parker  and  Haswell.) 


70  shows  in  longitudinal  section  the  cloacal  cavity  of  a  simple 
sponge,  at  the  bottom  of  which  are  the  openings  of  the  radial 
canals;  the  body  wall  is  seen  to  be  crowded  with  both  radial 
and  incurrent  canals,  which  have  been  cut  lengthwise.  The 
relations  of  the  various  canals  to  one  another  are  shown  in 
Figure  75;  here  the  arrows  indicate  the  direction  of  the  current 
L 


146 


AN  .INTRODUCTION  TO  ZOOLOGY 


of  water,  which  enters  the  incurrent  canal,  passes  through  the 
prosopyles  into  the  radial  canal,  and  thence  into  the  cloacal 
cavity,  finally  escaping  from  the  body  by  way  of  the  osculum. 

The  surface  area  of 
the  epithelium  cover- 
ing the  body,  and 
lining  the  internal 
cavities,  is  enormously 
increased  by  the  canal 
system. 

Grantia  is  an  animal 
possessing  an  outer 
dermal  layer  of  cells, 
an  inner  gastral  epi- 
thelium, and  a  middle 
region  containing  cells 
of  several  varieties 
(Fig.  71).  The  der- 
mal epithelium  (Fig. 
71,  ect.)  covers  the 
entire  outer  surface  of 
the  body,  and  lines 
the  incurrent  canals. 
It  is  composed  ex- 
ternally of  a  single 
layer  of  flat  cells. 
Wherever  prosopyles 

occur  connecting  the 
FIG.  71.  Part  of  the  body  of  Grantia.     ect.,  ecto-    incurrent      with      the 
derm;    end.,  entoderm ;    mes.,  mesoderm ;  , 

5#,spicules.  (From  Dahlgren  and  Kepner.)    radial  canals,  a  Single 

large   dermal    cell, 

termed  a  porocyte,  is  present.  The  porocytes  are  derived  from 
cells  of  the  dermal  epithelium.  They  are  large  and  granular,  and 
frequently  exhibit  ameboid  movements.  The  prosopyle  is  an 
intracellular  perforation  of  the  porocyte.  Cells,  called  sclero- 


SPONGES,  FLAT  WORMS,  AND   ROUND   WORMS        147 


blasts,  which  produce  spicules,  are  also  considered  constituents 
of  the  dermal  layer. 

The  inner  epithelium  lines  the  cloacal  cavity  and  the  radial 
canals.  In  the  latter  it  consists  of  one  layer  of  collared  flagel- 
lated cells  (Fig.  71,  end.),  termed  choanocytes.  No  collar  cells 
are  present  in  the 
epithelium  -lining  the 
cloacal  cavity.  The 
flagella  of  the  collar 
cells  create  the  cur- 
rent of  water  which 
is  continually  flowing 
through  the  body  wall 
into  the  cloacal  cavity 
and  out  of  the  oscu- 
lum. 

The  middle  region  of 
the  body  wall  is  not    e 
so    definite    nor    firm 
in  structure  as  are  the 
outer  and  inner  epi- 

FIG.  72.  Section  of  a  portion  of  Grantia.  I,  open- 
ings of  incurrent  canals ;  2,  incurrent  canal ; 
3,  prosopyle ;  4,  radial  canal ;  5,  choano- 
cytes ;  6,  spicules  ;  7,  opening  of  radial  canal. 
(From  Shipley  and  MacBride  after  Dendy.) 

FIG.  73.  Develop-  ,    .                                             .     .             .,        , 

ing  spicules  of  thelia'  but'  nevertheless,  it  is  considered 

a    sponge.  a  distinct  cellular  layer.     Ameboid  wan- 

(From   Dahl-  dering  cells,  which  ingest  food  or  act  as 

grenandKep-  storage  cells,  are  found  here,  as  well  as 

ner  after   A.   ^  reproductii}e  cells  which  always  arise 
Maas.)  .       ,          ...     . 

in  the  middle  layer. 

The  soft  body  wall  of  Grantia  (Fig.  72)  is  supported  and  pro- 
tected by  a  skeleton  composed  of  a  great  number  of  spicules  of 


148  AN  INTRODUCTION  TO  ZOOLOGY 

carbonate  of  lime.  Four  varieties  of  spicules  are  always  present, 
(i)  long  straight  monaxon  rods  guarding  the  osculum,  (2)  short 
straight  monaxon  rods  surrounding  the  incurrent  pores,  (3)  tri- 
radiate  spicules  always  found  embedded  in  the  body  wall,  and 
(4)  T-shaped  spicules  lining  the  cloacal  cavity;  four-  and  five- 
rayed  spicules  may  also  be  present.  Spicules  are  built  up  within 
cells  called  scleroblasts  (Fig.  73),  which  form  part  of  the  inner 
stratum  of  the  dermal  layer.  A  slender  organic  axial  thread  is 
first  built  up  within  the  cell;  around  this  is  deposited  the  calcare- 
ous matter;  the  whole  spicule  is  then  insheathed  by  an  envelope 
of  organic  matter  like  that  composing  the  axial  thread. 

Nutrition.  —  Grantia  lives  upon  the  minute  organisms  and 
small  particles  of  organic  matter  that  are  drawn  into  the  incur- 
rent canals  by  the  current  of  water  produced  by  the  beating  of 
the  collar-cell  cilia.  Some  of  the  food  particles  are  probably 
ingested  by  the  porocytes;  but  the  majority  of  them  are  engulfed 
by  the  collar  cells.  Digestion,  as  in  the  Protozoa,  is  intracellular, 
food  vacuoles  being  formed.  The  distribution  of  the  nutriment 
is  accomplished  by  the  passage  of  digested  food  from  cell  to  cell, 
aided  by  the  ameboid  wandering  cells  of  the  middle  layer. 

Excretory  matter  is  discharged  through  the  general  body  surface, 
assisted  probably  by  the  ameboid  wandering  cells,  and  possibly 
by  the  collar  cells  also.  Respiration  likewise  takes  place,  in  the 
absence  of  special  organs,  through  the  cells  of  the  body  wall. 

Sponges  do  not  possess  differentiated  nervous  organs,  but  are 
able  to  respond  to  certain  stimuli.  The  pores  and  oscula  are 
surrounded  by  contractile  cells,  called  myocytes,  which  are  able 
to  close  these  openings.  Apparently  these  cells  respond  to  direct 
stimulation,  since  no  nervous  tissue  is  present.  They,  therefore, 
represent  what  may  be  considered  the  very  beginning  of  a  neuro- 
muscular  mechanism  (130,  pp.  59-60). 

Reproduction.  —  Reproduction  in  Grantia  takes  place  by  both 
sexual  and  asexual  methods.  In  the  latter  case,  a  bud  arises 
near  the  point  of  attachment,  finally  becomes  free,  and  takes  up  a 
separate  existence. 


SPONGES,   FLAT  WORMS,  AND   ROUND   WORMS        149 

The  sexual  reproductive  cells  lie  in  the  middle  layer  of  the  body 
wall.  Both  eggs  and  sperms  occur  in  a  single  individual  i.e. 
Grantia  is  monoecious  or  hermaphroditic.  Spermato genesis  is 
probably  similar  to  that  of  an  allied  genus,  Sycon.  The  pri- 
mordial germ  cell,  called  a  spermatogonium,  divides,  producing  a 
covering  cell,  the  spermatocyst,  and  a  central  sperm  mother  cell, 
the  spermatocyte.  The  latter  forms  a  number  of  spermatids 
by  mitosis;  these  transform  into  spermatozoa.  The  ova  arise 
by  the  growth  of  certain  cells  of  the  middle  layer  which  are  nour- 
ished by  neighboring  cells.  In  Sycon,  two  polar  bodies  are 
formed;  the  sperm  penetrates  the  egg  just  before  the  formation 
of  the  second  polar  body. 

Embryology.  —  The  development  of  the  fertilized  egg  has  been 
observed  in  Sycon  (Fig.  74)  and  is  probably  similar  to  what  occurs 
in  Grantia.  The  egg  (a)  segments  by  three  vertical  divisions 
into  a  pyramidal  plate  of  eight  cells  (b,  c).  A  horizontal  division 
now  cuts  off  a  small  cell  from  the  top  of  each  of  the  eight,  the 
result  being  a  layer  of  eight  large  cells  crowned  by  a  layer  of  eight 
small  cells.  The  cells  now  become  arranged  about  a  central 
cavity,  producing  a  blastula-like  sphere  (d).  The  small  cells 
multiply  rapidly  and  develop  flagella,  while  the  large  cells  become 
granular.  The  small  cells  are  now  partially  grown  over  by  the 
others,  forming  a  structure  called  the  amphiblastula  (e).  The 
mass  of  cells  then  becomes  disk-shaped  by  the  pushing  in  of  the 
flagellated  cells  (/).  Two  layers  are  thus  formed,  between  which 
the  middle  layer  arises.  The  invaginated  side  soon  becomes  at- 
tached (g),  and  the  embryo  lengthens  into  a  cylinder  at  the  distal 
end  of  which  a  cavity,  the  osculum,  appears  (ti).  In  the  mean- 
time, spicules  and  the  canal  system  arise  in  the  body  wall. 

SPONGES  IN  GENERAL 

Grantia  has  served  as  a  type  of  the  Phylum  Porifera,  but  other 
sponges  vary  so  markedly  from  this  type,  that  a  general  survey 
of  the  entire  group  must  be  taken  before  their  resemblances  to 


AN  INTRODUCTION  TO  ZOOLOGY 


other  animals  can  be  discussed  successfully.  The  character  of 
the  object  to  which  sponges  are  attached  causes  them  to  assume 
exceedingly  irregular  shapes,  the  rocks  being  frequently  incrusted 


FIG.  74.  Development  of  a  simple  sponge  (Sycon  raphanus).  a,  ovum; 
b,  c,  ovum  segmented  ;  d,  blastula  ;  e,  amphiblastula  ;  /,  commencement 
of  invagination ;  g,  gastrula  attached ;  h,  i,  young  sponge.  (From 
Parker  and  Haswell  after  Schulze.) 


SPONGES,  FLAT  WORMS,  AND   ROUND  WORMS        151 


by  indefinite  masses  of  spongy  tissue.  This  makes  it  difficult  to 
decide  what  constitutes  an  individual  sponge.  Perhaps  the  best 
way  to  separate  one  from  another  is  to  consider  all  of  the  tissue 
surrounding  one  osculum  as  a  single  individual. 


c.c. 


B 

FIG.  75.  Types  of  canal  systems  of 
sponges.  The  arrows  indicate  the 
direction  of  the  current  of  water. 
The  thick  black  line  represents  the 
gastral  layer,  the  dotted  portion,  the 
dermal  layer,  ap.  p.,  apopyle ; 
ex.  c.,  excurrent  canal ;  fl.  c., 
flagellated  chamber  ;  G  C,  gastral 
cavity  (cloaca) ;  in.  r.,  incurrent 
canal ;  osc.,  osculum  ;  ost.,  ostia  ; 
pr.  p.,  prosopyle.  (After  Minchin 
in  Lankester's  Treatise.) 

Canal  System.  —  The  canal 
systems  of  sponges  may  be 
grouped  under  three  types;  the 
first  (Fig.  75,  A)  consists  of  in- 
current  pores  (/>),  a  gastral  cavity 
(GC),  and  an  osculum  (osc.}. 


152  AN  INTRODUCTION  TO  ZOOLOGY 

The  second  type  (Fig.  75,  B)  is  more  complicated;  the  water  flows 
through  the  dermal  pores  (ostia)  into  the  incurrent  canals  (inc.) ; 
then  through  the  chamber  pores  (prosopyles,  pr.  p.)  into  the  radial 
canals  (fl.  c.);  from  here  it  is  propelled  by  the  flagella  of  the 
choanocytes  into  the  gastral  cavity  (GC),  finally  passing  out 
through  the  osculum  (osc.).  In  the  third  type  (Fig.  75,  C)  there 
are  three  distinct  parts,  (i)  the  water  passes  through  the  dermal 
ostia  (ost.)  and  by  way  of  incurrent  canals  (inc.)  reaches  (2)  a 
number  of  small  chambers  (fl.c.)  lined  with  choanocytes,  thence  it 
is  carried  through  (3)  an  excurrent  system  (exc.)  to  the  gastral 
cavity  (GC),  and  finally  out  of  the  osculum. 

The  skeletons  of  sponges  are  composed  of  spicules  and  spongin. 
The  former  are  secreted  by  cells  of  the  dermal  layer,  and  consist 
of  calcite  and  silica.  The  spongin  is  an  organic  substance. 

Reproduction.  —  Reproduction  is  either  asexual  or  sexual. 
By  the  asexual  method  there  are  produced  buds  and  gemmules. 
Buds  may  be  set  free  to  take  up  a  separate  existence,  or  may 
remain  attached  to  the  parent  sponge,  aiding  in  the  formation  of 
a  complex  assemblage  of  individuals.  Gemmules  occur  in  the 
fresh-water  sponge,  Spongilla,  and  several  marine  species.  A 
number  of  cells  in  the  middle  layer  of  the  body  wall  gather  into 
a  ball  and  become  surrounded  by  protecting  spicules.  These 
gemmules  are  formed  in  the  autumn  just  before  the  death  of  the 
adult  sponge.  In  the  spring  they  develop  into  new  sponges. 
They  are  of  value  in  carrying  the  race  through  a  period  of  adverse 
conditions,  such  as  the  winter  season.  Sexual  reproduction  takes 
place  essentially  as  described  for  Sycon  on  page  149. 

Pieces  cr  sponges  are  capable  of  regenerating  entire  animals. 
This  characteristic  enables  sponge-growers  to  plant  a  bed  of 
sponges  by  scattering  small  pieces  over  the  bottom  of  the  sea  in 
favorable  places.  After  a  period  of  several  years,  animals  of 
commercial  value  may  be  gathered,  and  a  new  lot  of  "  slips  " 
set  out. 

Embryology.  —  The  principal  stages  of  sponge  embryology 
may  be  presented  briefly  as  follows:  — 


SPONGES,   FLAT  WORMS,  AND   ROUND   WORMS        153 

(1)  The  fertilized  egg  divides  to  form  a  group  of  cells. 

(2)  These  cells  become  separated  into  three  classes:  (a)  flagel- 
lated outer  cells,  (b)  large  non-flagellated  cells  either  within  or  at 
the  posterior  pole,  and  (c)  undifferentiated  cells  between  the  other 
two  varieties.     This  embryo  swims  about  with  the  aid  of  its 
flagella. 

(3)  The  larva  comes  to  rest,  and  the  flagellated  cells  pass  to 
the  interior,  while  the  large  non-flagellated  cells  migrate  to  the 
outside. 

(4)  The  flagellated  cells  become  the  choanocytes  of  the  adult; 
the  large  non-flagellated  cells  become  differentiated  into  the  three 
strata  of  the  dermal  layer;   and  the  cells  of  the  middle  region 
remain  as  the  ameboid  wandering  cells  and  reproductive  cells  of 
the  full-grown  sponge. 

2.  FLAT  WORMS  —  PL  AN  ARIA 
(Planar ia  maculata  Leidy) 

External  Features.  —  Planaria  maculata  (Fig.  76)  is  a  flat 
worm  found  only  in  fresh  water,  usually  clinging  to  the  underside 
of  logs  or  stones.  Like  most  of  the  members  of  the  Phylum 
Platyhelminthes,  its  body  is  extremely  flattened  dorso-ventrally, 
and  is  bilaterally  symmetrical.  Planaria  is  broad  and  blunt  at 
the  anterior,  and  pointed  at  the  posterior  end.  The  length  of  an 
adult  specimen  may  reach  half  an  inch.  The  body  contains  so 
much  coloring  matter  as  to  make  the  location  of  the  internal 


4 

FIG.  76.  Planaria  polychroa.     i,  eye ;    2,  side  of  head ;    j,  proboscis ; 
pharynx  sheath  ;  5,  genital  pore.     (From  Shipley  and  MacBride.) 


154 


AN  INTRODUCTION  TO  ZOOLOGY 


structures  difficult  to  determine  in  a  living  animal.     In  order  to 
study  Planaria  successfully  in  the  laboratory,  the  soft,  contractile 

body  is  usually  placed  on  a 
slide,  and  then  pressed  out 
slightly  with  a  cover  glass. 

A  pair  of  eye  spots  (Fig.  76,  /) 
are  present  on  the  dorsal  sur- 
face near  the  anterior  end. 
The  mouth  is  in  a  peculiar  posi- 
tion near  the  middle  of  the 
ventral  surface.  From  it  the 
muscular  proboscis  (j)  may 
extend.  Posterior  to  the 
mouth  is  a  smaller  opening, 
the  genital  pore  (5).  The  sur- 
face of  the  body  is  covered  with 
cilia,  which  propel  the  animal 
through  the  water.  This  is 
not  the  only  method  of  loco- 
motion, since  muscular  con- 
traction is  also  effective. 

Structure.  —  A  study  of  the 
structure  of  the  adult  and  of 
the  early  embryonic  stages 
shows  Planaria  to  be  a  triplo- 
blastic  animal  possessing  the 
three  germ  layers,  ectoderm, 
mesoderm,  and  entoderm,  from 
ovary;  i\  22,  wn^cn  several  systems  of  or- 

is,  branches  of  intestine ;  In,  lateral   gans      have      been      derived. 

nerve  ;  m,  mouth ;    ph,  pharynx  ;    There  are  well-developed  mus- 

nervouSj    digestive,    ex- 
, and     "productive 

mon  genital  pore.    (From  Lan-   systems;  these  are  constructed 

kester's  Treatise  after  V.  Graff.)     in  such  a  way  as  to  function 


FIG.  77.  Anatomy  of  Planaria. 
en,  brain  ;  e,  eye  ; 


od,  oviduct ;  t,  testis  ;  w,  uterus  ; 
v,  yolk  glands ;  vd,  vas  deferens ; 
§ ,  penis  ;  § ,  vagina  ;  § ,  <j> ,  com- 


SPONGES,   FLAT  WORMS,  AND   ROUND   WORMS        155 

without  the  coordination  of  a  circulatory  system,  respiratory 
system,  ccelom,  and  anus. 

DIGESTIVE  SYSTEM. — The  digestive  system  (Fig.  77)  consists 
of  a  mouth  (m.),  a  pharynx  (ph.)  lying  in  a  muscular  sheath,  and 
an  intestine  of  three  main  trunks  (i}  i2,  is)  and  a  large  number  of 
small  lateral  extensions.  The  muscular  pharynx  can  be  ex- 
tended as  a  proboscis  (Fig.  76,  j) ;  this  facilitates  the  capture  of 
food.  Digestion  is  both  intercellular  and  intracellular,  i.e.  part 
of  the  food  is  digested  in  the  intestinal  trunks  by  secretions  from 
cells  in  their  walls;  whereas  other  food  particles  are  engulfed 
by  pseudopodia  thrust  out  by  cells  lining  the  intestine,  and  are 
digested  inside  of  the  cells  in  vacuoles.  The  digested  food  is  ab- 
sorbed by  the  walls  of  the  intestinal  trunks,  and,  since  branches 
from  these  penetrate  all  parts  of  the  body,  no  circulatory  system 
is  necessary  to  carry  nutriment  from  one  place  to  another.  As 
in  Hydra,  no  anus  is  present,  the  faces  being  ejected  through  the 
mouth. 

EXCRETORY  SYSTEM.  —  The  excretory  system  comprises  a  pair 
of  longitudinal,  much-coiled  tubes,  one  on  each  side  of  the  body; 
these  are  connected  near  the  anterior  end  by  a  transverse  tube, 
and  open  to  the  exterior  by  two  small  pores  on  the  dorsal  surface. 
The  longitudinal  and  transverse  trunks  give  off  numerous  finer 
tubes  which  ramify  through  all  parts  of  the  body,  usually  ending 
in  a  flame  cell.  The  flame  cell  (Fig.  78)  is  large  and  hollow, 
with  a  bunch  of  flickering  cilia  (c)  extending  into  the  central  cav- 
ity (e).  Since  it  communicates  only  with  the  excretory  tubules 
it  is  considered  excretory  in  function,  though  it  may  also  carry 
on  respiratory  activities. 

MUSCULAR  SYSTEM.  —  The  power  of  changing  the  shape  of  its 
body,  which  may  be  observed  when  Planaria  moves  from  place 
to  place,  lies  principally  in  three  sets  of  muscles,  a  circular  layer 
just  beneath  the  ectoderm,  external  and  internal  layers  of  longi- 
tudinal muscle  fibers,  and  a  set  of  oblique  fibers  lying  in  the  meso- 
derm. 

NERVOUS  SYSTEM.  —  Planaria  possesses  a  well-developed  nerv- 


156 


AN  INTRODUCTION  TO  ZOOLOGY 


ous  system,  consisting  of  a  bilobed  mass  of  tissue  just  beneath  the 
eye  spots,  called  the  brain  (Fig.  77,  en)  and  two  lateral  longitu- 

dinal nerve  cords  (In)  connected 
by  transverse  nerves.  From  the 
brain,  nerves  pass  to  various  parts 
of  the  anterior  end  of  the  body, 
imparting  to  this  region  a  highly 
sensitive  nature. 

REPRODUCTIVE  SYSTEM.  —  Re- 
production is  by  fission  or  by  the 
sexual  method.  Each  individual 
possesses  both  male  and  female 
organs,  i.e.  is  hermaphroditic. 
The  male  organs  may  be  located 
easily  in  Figure  77;  they  consist 
of  numerous  spherical  testes  (/.) 
connected  by  small  tubes  called 
vasa  deferentia  (vd.);  the  vas 

FIG.  78.   Flame   cell  of  Planaria.   deferens   from   each    side   of    the 

bod    joins  ^  ^^  Qr        ^  (  $  } 

• 
a    muscluar    organ  which  enters 

the  genital  cloaca.  A  seminal 
vesicle  lies  at  the  base  of  the  penis,  also  a  number  of  unicellular 
prostate  glands.  Spermatozoa  originate  in  the  testes,  and  pass, 
by  way  of  the  vasa  deferentia,  into  the  seminal  vesicle,  where 
they  remain  until  needed  for  fertilization.  The  female  repro- 
ductive organs  comprise  two  ovaries  (g),  two  long  oviducts  (od.) 
with  many  yolk  glands  (v)  entering  them,  a  vagina  (  $  )  ,  which 
opens  into  the  genital  cloaca,  and  the  uterus,  which  is  also  con- 
nected with  this  cavity.  The  eggs  originate  in  the  ovary,  pass 
down  the  oviduct,  collecting  yolk  from  the  yolk  glands  on  the 
way,  and  finally  reach  the  uterus.  Here  fertilization  occurs,  and 
cocoons  are  formed,  each  containing  from  four  to  more  than  twenty 
eggs,  surrounded  by  several  hundred  yolk  cells.  The  develop- 
ment of  the  egg  is  illustrated  and  explained  in  Figure  79. 


c,  cilia  ;  e,  opening  into  excre- 
tory  tubule.  (From  Lankes- 
ter's  Treatise  ) 


vy 

B 


O    0 


FIG.  80.  Regeneration  of  Planaria  maculata.  A,  normal  worm ;  B,  B1,  regeneration  of 
anterior  half ;  C,  C1,  regeneration  of  posterior  half ;  D,  crosspiece  of  worm  ;  D1, 
D2,  D3,  D4,  regeneration  of  same  ;  E,  old  head  ;  E1,  E2,  E3,  regeneration  of  same ;  F, 
F1,  regeneration  of  new  head  on  posterior  end  of  old  head.  (From  Morgan.) 


SPONGES,  FLAT  WORMS,  AND  ROUND   WORMS        157 


FIG.  79.  Development  of  Planaria  lactea.  i,  egg  (0)  surrounded  by  yolk  (v)  ; 
2,  four  blastomeres  from  segmented  egg ;  j,  later  stage ;  more  blasto- 
meres  (bl) ;  4,  much  later  stage,  differentiation  of  blastomeres  into  ecto- 
derm (ep\  entoderm  (hy),  a  provisional  pharynx  (pti),  and  wandering 
cells  (w) ;  5,  cellular  differentiation  more  advanced;  ep,  ectoderm ; 
ent,  primitive  gut ;  hy,  entoderm ;  ph,  pharynx ;  6,  embryo  changes 
shape  to  a  flattened  ovoid,  ent,  primitive  gut:  m,  mouth;  ^phar- 
ynx. (From  Lankester  after  Hallez.) 

Regeneration.  —  Planarians  show  remarkable  powers  of  re- 
generation. If  an  individual  is  cut  in  two  (Fig.  80,  A),  the  an- 
terior end  will  regenerate  a  new  tail  (B,  B'),  while  the  posterior 
part  develops  a  new  head  (C,  CO-  A  crosspiece  (D)  will  regener- 
ate both  a  head  at  the  anterior  end,  and  a  new  tail  at  the  posterior 
end  (D'-D4).  The  head  alone  of  a  Planarian  will  grow  into  an 
entire  animal  (E — E3).  Pieces  cut  from  various  parts  of  the 
body  will  also  regenerate  completely.  No  difficulty  is  experi- 
enced in  grafting  pieces  from  one  animal  upon  another,  and  many 
curious  monsters  have  been  produced  in  this  way. 


158  AN  INTRODUCTION  TO  ZOOLOGY 

FLAT  WORMS  IN  GENERAL 

Flat  worms  are  usually  separated  into  three  classes. 

Class  i. — Turbellaria.  Flat  worms  with  ciliated  epidermis 
and  digestive  cavity;  mostly  non-parasitic.  Example,  Planaria. 

Class  2.  —  Trematoda.  Flat  worms  without  ciliated  epi- 
dermis; digestive  apparatus  well  developed;  ecto-  or  endo- 
parasitic.  Example,  liver  fluke  of  sheep. 

Class  3.  —  Cestoda.  Flat  worms  without  ciliated  epidermis 
and  digestive  apparatus;  endoparasitic.  Example,  tapeworm. 

Planaria  shows  the  principal  features  characteristic  of  flat 
worms;  but  there  must  necessarily  be  wide  diversity  in  structure 
among  the  members  of  a  phylum  composed  of  both  free-living 
and  parasitic  forms.  The  free-living  flat  worms,  such  as  Planaria, 
probably  are  more  nearly  like  the  ancestors  of  this  phylum  than 
the  parasitic  species,  since  the  latter  have  undoubtedly  become 
degenerate  with  respect  to  certain  structures,  and  more  special- 
ized with  respect  to  others,  because  of  their  modified  habits  of 
life.  From  a  study  of  Planaria,  therefore,  we  can  gain  some  idea 
of  what  kind  of  an  animal  gave  rise  to  the  flat  worms. 

In  the  first  place  definite  bilateral  symmetry  is  exhibited  here. 
Flat  worms  thus  show  an  advance  in  this  respect  over  the  more 
simple  radial  symmetry  of  Ccelenterates.  A  second  point  to  be 
noted  is  the  presence  of  a  distinct  mesoderm  between  the  ecto- 
derm and  entoderm.  This  mesoderm  consists  of  muscle  cells 
and  connective  tissue.  As  in  the  Coelenterates,  however,  there 
is  but  one  body  cavity,  represented  by  the  digestive  system, 
though  the  genital  sacs  may  represent  a  second  cavity,  known 
as  the  ccelom,  which  is  well  developed  in  more  complex 
animals. 

The  digestive  apparatus  is  not  a  simple  blind  sac,  as  in  Hydra, 
but  consists  of  several  large  branches,  each  with  many  smaller 
side  pouches  entering  it,  the  whole  being  modified  to  transport 
nutriment  to  all  parts  of  the  body,  a  circulatory  system  being 
lacking.  An  anus,  however,  is  absent,  the  ingestion  of  food  and 


SPONGES,  FLAT  WORMS,  AND   ROUND   WORMS        159 

the  egestion  of  faeces  taking  place  through  a  single  aperture,  the 
mouth. 

The  nervous  system  of  flat  worms  shows  a  marked  advance, 
especially  in  concentration,  over  that  of  the  Coelenterates.  The 
presence  of  a  brain  near  the  end  of  the  body  directed  forward  in 
moving  is  what  would  be  expected,  since  this  end  receives  all 
sensations  first,  and  nerve  cells  would  be  developed  in  the  region 
of  greatest  stimulation. 


TABLE  V 

THE  CHARACTERS  OF  HYDRA  AND  PLANARIA  CONTRASTED 


CHARACTER 


HYDRA 


PLANARIA 


Symmetry 
Germ  layers 
Digestive  system 

Coelom 
Excretory  system 


Nervous  system 
Muscular  system 
Reproductive  system 


Radial 
Diploblastic 
Coelenteric  cavity 

Absent 
None 


Network  of  nerve  cells 


Processes  of  ectoderm 
and  entoderm  cells 


No  accessory  reproduc- 
tive organs 


Bilateral 
Triploblastic 

Pharynx  and  branched 
intestine 

May  be  represented  by 
genital  sacs 

Complicated  system  of 
tubes,  ending  in  flame 
cells  and  opening  to 
exterior 

Nerve  cells  concentrated 
into  brain  and  nerve 
cords 

Muscle  fibers  with  no 
other  function,  from 
mesoderm  cells 

A  complicated  reproduc- 
tive apparatus 


160  AN  INTRODUCTION  TO  ZOOLOGY 

Excretory  organs  and  a  complicated  reproductive  system  make 
their  appearance  in  this  phylum.  The  muscle  fibers  of  Hydra 
are  simple  specializations  of  cells  that  perform  other  functions; 
but  in  Planaria,  cells  are  set  aside  for  no  other  purpose  than  to 
give  a  high  degree  of  contractility  to  the  body.  These  cells 
form  distinct  bands  of  circular,  longitudinal,  and  oblique  muscles. 
Table  V  contrasts  the  structures  of  Hydra  and  Planaria.  In 
this  way  a  clear  idea  of  the  advance  in  complexity  exhibited 
by  the  latter  may  be  gained. 

3.  ROUND  WORMS  —  ASCARIS 
(Ascaris  lumbricoides  Linn.) 

External  Features.  —  Ascaris  is  a  round  worm  parasitic  in  the 
intestines  of  pigs,  horses,  and  man.  The  sexes  are  separa.;, 
The  female,  being  the  larger,  measures  from  five  to  eleven  inches 
in  length  and  about  one  fourth  of  an  inch  in  diameter.  The  body 
is  light  brown  in  color;  it  has  a  dorsal  and  a  ventral  white  narrow 
stripe  running  its  entire  length  and  a  broader  lateral  line  is  present 
on  either  side.  The  anterior  end  possesses  a  mouth  opening, 
surrounded  by  one  dorsal  and  two  ventral  lips.  Near  the  pos- 
terior end  is  the  anal  opening,  from  which,  in  the  male,  extends 
penial  seta  of  use  during  copulation.  The  male  can  be  distin- 
guished from  the  female  by  the  presence  of  a  bend  in  the  posterior 
part  of  the  body. 

Internal  Anatomy.  —  If  an  animal  is  cut  open  along  the  dorsal 
line  (Fig.  81),  it  will  be  found  to  contain  a  straight  alimentary 
canal,  and  certain  other  organs,  lying  in  a  central  cavity,  the 
ccdom,  a  cavity  met  with  now  for  the  first  time.  The  alimentary 
canal  (2)  is  very  simple,  since  the  food  is  taken  from  material 
already  digested  by  the  host  whose  intestine  the  worm  inhabits. 
It  opens  at  the  posterior  end  through  the  anus,  which  is  not  present 
in  members  of  the  phyla  already  discussed.  A  muscular  pharynx 
(i)  draws  the  fluids  into  the  long  non-muscular  intestine  (2)- 


SPONGES,   FLAT  WORMS,  AND  ROUND   WORMS        161 


through  the  walls  of  which  the 
nutriment  is  absorbed.  Just  be- 
fore the  anal  opening  is  reached, 
the  intestine  gradually  becomes 
smaller;  this  portion  is  known  as 
the  rectum. 

The  excretory  system  consists  of 
two  longitudinal  canals  (Fig.  81, 
7),  one  in  each  lateral  line;  these 
open  to  the  outside  by  a  single 
pore  (8)  situated  near  the  anterior 
end  in  the  ventral  body  wall. 

A  ring  of  nervous  tissue  sur- 
rounds the  pharynx  and  gives  off 
two  large  nerve  cords,  one  dorsal, 
the  other  ventral,  and  a  number 
of  other  smaller  strands  and 
connections. 

The  male  reproductive  organs 
are  a  single  coiled  thread-like 
testis,  from  which  a  vas  deferens 
leads  to  a  wider  tube,  the  seminal 
vesicle;  this  is  followed  by  the 
short  muscular  ejaculatory  duct 
which  opens  into  the  rectum. 
In  the  female  lies  a  Y-shaped 
reproductive  system.  Each 
branch  of  the  Y  consists  of  a 
coiled  thread-like  ovary  (Fig.  81, 
3)  which  is  continuous  with  a 
larger  canal,  the  uterus  (4).  The 
uteri  of  the  two  branches  unite 
into  a  short  muscular  tube,  the 
vagina  (5),  which  opens  to  the  out- 
side through  the  genital  aperture 
(6).  Fertilization  takes  place  in 
che  uterus.  The  egg  is  then  sur- 
rounded by  a  shell  of  chitin,  and 


162 


AN  INTRODUCTION  TO  ZOOLOGY 


passes  out  through  the  genital  pore.     The  chitinous   eggshell 
prevents  the  digestion  of  the  egg  within  the  intestine  of  the  host. 

int 


loll 


7TV 


FIG.  82.  Transverse  section  of  Ascaris  lumbricoides.  en.,  cuticle;  dL, 
dorsal  line;  der.  epthm.,  epidermis;  ex.  v.,  excretory  tube;  int.,  intes- 
tine; lat.  I.,  lateral  line;  m.,  muscular  layer  ;  ovy.,  ovary  ;  «/.,  uterus; 
v.  v.,  ventral  line.  (From  Parker  and  Haswell  after  Vogt  and  Yung.) 

The  relations  of  the  various  organs  to  one  another,  as  well  as 
the  structure  of  the  body  wall,  and  the  character  of  the  ccelom, 
are  shown  in  Figure  82,  which  is  a  transverse  section  of  a  female 
specimen  of  Ascaris  lumbricoides.  The  body  of  the  worm  should 
be  considered  as  consisting  of  two  tubes,  one,  the  intestine  (int.), 
lying  within  the  other,  the  body  wall;  while  between  them  is  a 
cavity,  the  ccelom,  in  which  lie  the  reproductive  organs  (ovy.  and 

*.). 

The  body  wall  is  composed  of  several  layers,  an  outer  chitinous 
cuticle  (cu.),  a  thin  layer  of  ectoderm  (der.  epthm.)  just  beneath  it, 
and  a  thick  stratum  of  longitudinal  muscle  fibers  (m.),  mesodermal 
in  origin,  lining  the  ccelom.  Thickenings  of  the  ectoderm  form 
the  dorsal  (d.  1),  ventral  (v.  v)  and  lateral  (lat.  I)  lines.  In  each 


SPONGES,  FLAT  WORMS,   AND   ROUND    WORMS        163 


of  the  last-named  lies  one  of  the  longitudinal  excretory  tubes 
(ex.  v.).     The  nerve  cords  are  also  embedded  in  the  body  wall. 

The  intestine  consists  of  a  single  layer  of  columnar  cells,  the 
entoderm,  coated  both  within  and  without  by  a  thin  cuticle. 

The  codom  of  Ascaris  differs  from  that  of  the  higher  animals  in 
several  respects.  Typically  the  ccelom  is  a  cavity  in  the  meso- 
derm  lined  by  an  epithelium;  into  it  the  excretory  organs  open, 
and  from  its  walls  the  reproductive  cells  originate.  In  Ascaris 
the  so-called  ccelom  is  lined  only  by  the  mesoderm  of  the  body 
wall,  there  being  no  mesoderm  surrounding  the  intestine.  Fur- 
thermore, the  excretory  organs  open  to  the  exterior  through  the 
excretory  pore,  and  the  reproductive  cells  are 
not  derived  from  the  coclomic  epithelium. 
The  body  cavity  of  Ascaris,  therefore,  dif- 
fers structurally  and  functionally  from  that 
of  a  true  ccelom,  but  nevertheless  is  similar 
in  many  respects. 

ROUND  WORMS  IN  GENERAL 

The  round  worms  belong  to  the  Phylum 
Nemathelminthes.  They  are  mostly  parasitic ; 
only  a  comparatively  small  number  are  free- 
living,  inhabiting  damp  earth  or  fresh  and 
salt  water.  Among  the  most  interesting 
round  worms  is  the  parasite,  Trichina  spiralis 
(Fig.  83),  which  is  often  found  encysted  in 
the  muscles  of  the  pig.  When  insufficiently 
cooked  pork  infested  by  Trichina  is  eaten  by 
man,  the  young  parasites  become  mature  in 
the  intestine,  and  burrow  through  its  walls, 
causing  a  disease  known  as  Trichinosis. 


,  83.  Trichina 
spiralis  encysted 
among  muscle 
fibers.  (From 
Shipley  and 
MacBride  after 
Leuckart.) 


CHAPTER  X 
HE    EARTHWORM    AND    ANNELIDS    IN    GENERAL 

i.  THE  EARTHWORM 

(Lumbricus  terrestris  Linnaeus) 

THE  earthworm,  Lumbricus  terrestris,1  lives  in  the  ground  where 
the  soil  is  not  too  dry  or  sandy,  coming  to  the  surface  only  at 
night  or  after  a  rain.  The  burrows  are  cylindrical  and  penetrate 
to  a  depth  of  from  six  inches  to  several  feet.  Evidences  of  the 
presence  of  earthworms  are  found  in  the  little  heaps  of  black 
earth,  called  "  castings,"  which  strew  the  ground,  being  espe- 
cially noticeable  early  in  the  morning.  These  castings  are  really 
the  faeces  cast  out  from  the  alimentary  canal  of  the  worm.  Dar- 
win (139)  estimated  that  more  than  eighteen  tons  of  earthy  cast- 
ings are  carried  to  the  surface  in  a  single  year  on  one  acre  of 
ground,  and  in  twenty  years  a  layer  three  inches  thick  would  be 
transferred  from  the  subsoil  to  the  surface.  This  continuous 
honeycombing  of  the  soil  makes  the  land  more  porous,  and  insures 
the  better  penetration  of  moisture,  and  the  continuous  working 
over  of  the  surface  layers  of  earth  also  helps  to  make  the  soil  more 
fertile. 

External  Features.  —  Lumbricus  terrestris  reaches  a  length  of 
from  six  inches  to  as  much  as  a  foot.  It  is  capable  of  pronounced 
extensions  and  contractions,  so  that  the  length  of  an  individual 
varies,  as  does  also  its  diameter.  Although  its  body  is  cylindroid 
it  is  possible  to  recognize  dorsal,  ventral,  and  lateral  surfaces. 
Movement  takes  place  in  a  definite  direction,  the  advancing 

1  This  is  one  of  many  species  of  earthworms.  In  many  parts  of  this 
country  the  species  Allolobophora  (Helodrilus)  longa  or  one  of  the  species  of 
Diplocardia  are  more  abundant  in  cultivated  soiL 

164 


THE  EARTHWORM   AND  ANNELIDS  IN   GENERAL        165 


portion  being  known  as  the  anterior,  the  hinder  portion  as  the 
posterior  end.  The  principal  axis  of  the  body  is  antero-posterior, 
and,  since  the  chief  organs  both  external  and  internal  lie  half  on 
one  side  and  half  on  the  other  side  of  a  median  plane,  the  worm  is 
said  to  be  bilaterally  symmetrical. 

A  noticeable  feature  of  the  earthworm  is  its  division  into  a  large 
number  of  similar  rings  by  grooves  extending  around  the  body  at 
short  intervals.  These  rings  are  termed 
segments,  somites,  or  metameres,  and  the 
body  is  said  to  be  segmented  or  to  have 
a  metameric  structure.  The  earthworm 
and  many  of  its  near  relatives  differ  in 
this  respect  from  the  unsegmented  flat 
worms  and  round  worms,  such  as 
Planaria  and  Ascaris.  The  somites  are 
not  exactly  alike.  The  anterior  lobe 
extending  above  and  beyond  the  mouth, 
and  backward  on  the  dorsal  surface  in- 
tersecting the  first  segment  is  not  a  true 
somite;  it  is  known  as  the  prostomium. 
It  has  been  found  that  not  only  external 
structures,  but  also  internal  organs,  bear 
a  constant  relation  to  the  segments; 
for  this  reason  the  somites  have  been  numbered,  beginning  with 
the  one  just  back  of  the  prostomium.  In  mature  worms  the 
six  or  seven  somites,  XXXI  or  XXXII  to  XXXVII,  are  swollen 
on  their  dorsal  and  lateral  surfaces,  producing  a  saddle-shaped 
enlargement  known  as  the  clitellum,  of  use  during  reproduction. 

If  an  apparently  smooth  earthworm  is  drawn  through  the 
fingers  from  its  anterior  end  toward  the  tail,  it  feels  rough  to  the 
touch.  This  is  caused  by  small  /-shaped  chitinous  bristles,  called 
setce  (Fig.  84),  four  pairs  of  which  extend  outward  from  epidermal 
sacs  in  every  somite  except  the  first  and  last.  Each  pair  may  be 
moved  by  retractor  and  protractor  muscles.  The  arrangement 
of  the  setae  is  shown  in  the  tigh^-nand  half  of  the  section  in  Figure 


V  \J 

FIG.  84.  Two  setae  of  Lum- 
bricus,  highly  magni- 
fied. (From  Parker 
and  Haswell.) 


i66 


AN  INTRODUCTION  TO  ZOOLOGY 


85,  set.  New  setae  may  be  produced  by  any  one  of  a  number  of 
cells  lying  at  the  bottom  of  the  epidermal  sacs.  On  mature 
worms  the  setae  on  somite  XXVI  are  enlarged  and  modified  as 
sexual  setae. 


dors.V 


Tiep 


nephrost 


n.co 


set 


vent.r 


FIG.  85.  Transverse  section  through  the  middle  region  of  the  body  of  Lum- 
bricus.  circ.  mus.,  circular  muscle  fibers  ;  ccel.,  ccelom  ;  dors,  v.,  dorsal 
vessel;  epid.,  epidermis ;  ext.  neph.,  nephridiopore ;  hep.,  chloragogen 
cells;  long,  mus.,  longitudinal  muscles;  neph.,  nephridium ;  nephrost., 
nephrostome ;  n.  co.,  nerve  cord;  set.,  setae;  sub.  n.  vess.,  sub-neural 
vessel ;  typh.,  typhlosole  ;  vent,  v.,  ventral  vessel.  (From  Parker  and 
Haswell  after  Marshall  and  Hurst.) 

The  outer  covering  of  the  body  is  a  thin  transparent  membrane, 
the  cuticle  (Fig.  85,  cut.),  which  is  secreted  by  the  cells  lying  just 
beneath  it.  The  cuticle  protects  the  body  wall  from  physical 


THE   EARTHWORM  AND  ANNELIDS  IN   GENERAL      167 

or  chemical  injury;  numerous  fine  pores  allow  the  secretions  from 
unicellular  glands  to  pass  through  (Fig.  86).  Under  the  micro- 
scope it  is  seen  to  be  marked  with  very  fine  stria  which  cross  one 
another;  they  cause  the  surface  of  the  body  to  appear  iridescent. 
A  number  of  external  openings  of  various  sizes  allow  the  en- 
trance of  food  into  the  body,  and  the  exit  of  faeces,  excretory  prod- 
ucts, reproductive  cells,  etc.  (i)  The  mouth  is  a  crescentic 


FIG.  86.  Vertical  section  of  a  bit  of  epidermis  of  the  earthworm.  Four 
mucus  cells  in  different  stages  of  secretion.  Mucus  is  passing  through 
one  of  the  two  pores  in  the  cuticle,  cu.  (From  Dahlgren  and  Kepner.) 

opening  situated  in  the  ventral  half  of  the  first  somite;  it  is  over- 
hung by  the  prostomium.  (2)  The  oval  and  aperture  lies  in  the 
last  somite.  (3)  The  openings  of  the  sperm  ducts  or  vasa  deferen- 
tia  are  situated  one  on  either  side  of  somite  XV.  They  have 
swollen  lips;  a  slight  ridge  extends  posteriorly  from  them  to  the 
clitellum.  (4)  The  openings  of  the  oviducts  are  small  round  pores 
one  on  either  side  of  somite  XIV;  eggs  pass  out  of  the  body 
through  them.  (5)  The  openings  of  the  seminal  receptacles 
appear  as  two  pairs  of  minute  pores  concealed  within  the  grooves 
which  separate  somites  IX  and  X,  and  X  and  XI.  (6)  A  pair  of 
nephridiopores  (Fig.  85,  ext.  neph.),  the  external  apertures  of  the 
excretory  organs,  open  on  every  somite  except  the  first  three 
and  the  last.  They  are  usually  situated  immediately  anterior  tc 
the  outer  seta  of  the  inner  pair.  (7)  The  body  cavity  (Fig.  85 


1 68  AN  INTRODUCTION  TO  ZOOLOGY 

coel.)  communicates  with  the  exterior  by  means  of  dorsal  pores. 
One  of  these  is  located  in  the  mid-dorsal  line  at  the  anterior 
edge  of  each  somite  from  VIII  or  IX  to  the  posterior  end  of  the 
body. 

General  Internal  Anatomy.  —  If  a  specimen  is  cut  open  from 
the  anterior  to  the  posterior  end  by  an  incision  passing  through 
the  body  wall  a  trifle  to  one  side  of  the  mid-dorsal  line,  a  general 
view  of  the  internal  structures  may  be  obtained  (Fig.  87).  As  in 
Ascaris,  the  body  is  essentially  a  double  tube  (Fig.  85),  the  body 
wall  constituting  the  outer,  the  straight  alimentary  canal,  the 
inner;  between  the  two  is  a  cavity,  the  ccdom.  The  external 
segmentation  corresponds  to  an  internal  division  of  the  coelomic 
cavity  into  compartments  by  means  of  partitions,  called  septa, 
which  lie  beneath  the  grooves.  These  septa  are  absent  in  Ascaris. 
The  alimentary  canal  passes  through  the  center  of  the  body,  and  is 
suspended  in  the  coelom  by  the  partitions.  Septa  are  absent 
between  somites  I  and  II,  and  incomplete  between  somites  III 
and  IV,  and  XVII  and  XVIII.  The  walls  of  the  coelom  are  lined 
with  an  epithelium,  termed  the  peritoneum.  The  ccelomic  cavity 
is  filled  with  a  colorless  fluid  which  flows  from  one  compartment 
to  another  when  the  body  of  the  worm  contracts.  In  somites  IX 
to  XVI  are  the  reproductive  organs  (Fig.  93) ;  running  along  the 
upper  surface  of  the  alimentary  canal  is  the  dorsal  blood  vessel 
(Fig.  85,  dors.  v.);  and  just  beneath  it  lie  the  ventral  blood  vessels 
(vent,  v.)  and  nerve  cord  (n.  co.). 

Body  Wall.  —  The  strata  of  the  body  wall  are  as  follows 
(Fig.  85):  (i)  an  outer  noncellular  covering,  the  cuticle  (cut.); 
(2)  an  epidermis  (epid.)  composed  of  two  cellular  layers,  an  outer 
stratum  consisting  of  gland,  interstitial,  and  sense  cells,  and  an 
inner  stratum  of  very  small  cells;  (3)  a  layer  of  circular  muscle 
fibers  (arc.  mus.)  running  around  the  body,  each  fiber  being  long, 
pointed  at  both  ends,  and  longitudinally  striated;  (4)  a  thick 
layer  of  muscle  fibers  running  lengthwise  of  the  body  and  appear- 
ing in  cross  section  to  be  arranged  in  featherlike  groups  (long. 
mus.) ;  (5)  the  innermost  layer,  the  coelomic  epithelium  or  perito- 


THE   EARTHWORM   AND   ANNELIDS  IN   GENERAL       169 

neum,  a  thin  stratum  of  flattened  cells  lining  the  coelom.  The 
gland  cells  of  the  epidermis  (Fig.  86)  are  called  "  goblet  cells  " 
because  of  their  shape;  they  secrete  the  mucus  which  helps  to 
keep  the  surface  of  the  worm  moist. 

Digestive  System.  —  The  alimentary  canal  (Fig.  87)  consists 
of  (i)  a  mouth  cavity  or  buccal  pouch  in  somites  I  to  III,  (2)  a 
thick  muscular  pharynx  (ph.)  lying  in  somites  IV  and  V,  (3)  a 
narrow  straight  tube,  the  oesophagus  (ozs.)  which  extends  through 
somites  VI  to  XIV,  (4)  a  thin-walled  enlargement,  the  crop  or 
proventriculus  (cr.),  in  somites  XV  and  XVI,  (5)  a  thick  mus- 
cular-walled gizzard  (giz.)  in  somites  XVII  and  XVIII,  and  (6)  a 
thin- walled  intestine  (int.)  extending  from  somite  XIX  to  the  anal 
aperture.  The  intestine  is  not  a  simple  cylindrical  tube;  but 
its  dorsal  wall  is  infolded,  forming  a  longitudinal  ridge,  the 
typhlosole  (Fig.  85,  typh.).  This  increases  the  digestive  surface. 

The  wall  of  the  intestine  is  composed  of  five  layers:  (i)  an  inner 
lining  of  ciliated  epithelium,  (2)  a  vascular  layer  containing  many 
small  blood  vessels,  (3)  a  thin  layer  of  circular  muscle  fibers,  (4)  a 
layer  consisting  of  a  very  few  longitudinal  muscle  fibers,  and 
(5)  an  outer  thick  coat  of  chlorogogen  cells  (Fig.  85,  hep.)  modified 
from  the  coelomic  epithelium.  The  last-named  cells  also  cover  the 
dorsal  trunk  of  the  vascular  system  and  extend  down  into  the 
typhlosole.  The  function  of  the  chlorogogen  cells  is  not  certainly 
known,  but  it  has  been  suggested  that,  since  they  are  present  about 
the  alimentary  canal,  in  the  typhlosole,  and  in  close  proximity 
to  the  dorsal  blood  vessel  which  carries  the  food  after  absorption, 
they  probably  aid  in  the  elaboration  of  food.  That  they  have 
an  excretory  function  also  seems  probable,  since  chlorogogen 
granules  are  present  in  the  coelomic  fluid  of  adult  worms,  make 
their  way  through  the  body  wall,  especially  through  the  dorsal 
pores,  and  pass  outside  of  the  body  in  the  mucus  (154).  They 
also  disintegrate  in  the  coelomic  fluid,  and  the  waste  products 
are  eliminated  through  the  excretory  organs. 

At  the  sides  of  the  oesophagus  are  three  pairs  of  calciferous 
glands  (Fig.  87,  ces.  gl.),  one  pair  in  each  of  the  somites  from  X  to 


170  AN  INTRODUCTION  TO  ZOOLOGY 

XII.  The  first  pair  are  pouches  pushed  out  from  the  alimentary 
canal  and  opening  directly  into  the  oesophagus.  The  other  two 
pairs  are  swellings  of  the  cesophageal  wall ;  they  have  a  number  of 
small  cavities  which  open  directly  through  the  epithelium  into 
the  oesophagus  in  somite  XIV.  Carbonate  of  lime  is  produced 
by  these  glands,  and  poured  into  the  alimentary  canal,  where  it 
probably  neutralizes  acid  foods  (143). 

Nutrition.  —  FOOD.  —  The  food  of  the  earthworm  consists 
principally  of  pieces  of  leaves  and  other  vegetation,  particles  of 
animal  matter,  and  soil.  This  material  is  gathered  at  night.  At 
this  time  the  worms  are  active;  they  crawl  out  into  the  air,  and, 
with  their  tails  holding  fast  to  the  tops  of  their  burrows,  explore 
the  neighborhood. 

INGESTION.  —  Food  particles  are  drawn  into  the  buccal  cavity 
by  suction  produced  when  the  pharyngeal  cavity  is  enlarged. 
This  is  accomplished  by  the  contraction  of  the  muscles  which 
extend  from  the  pharynx  to  the  body  wall. 

DIGESTION.  —  In  the  pharynx,  the  food  receives  a  secretion  from 
the  pharyngeal  glands;  it  then  passes  through  the  oesophagus 
to  the  crop,  where  it  is  stored  temporarily.  In  the  meantime 
the  secretion  from  the  calciferous  glands  in  the  oesophageal  walls 
is  added,  neutralizing  the  acids.  The  gizzard  is  a  grinding  or- 
gan; in  it  the  food  is  broken  up  into  minute  fragments  by  being 
squeezed  and  rolled  about.  Solid  particles,  such  as  rough  pebbles, 
which  are  frequently  swallowed,  probably  aid  in  this  grinding 
process.  The  food  then  passes  on  to  the  intestine,  where  most  of 
the  digestion  and  absorption  takes  place. 

Digestion  in  the  earthworm  is  very  similar  to  that  of  higher 
animals.  The  digestive  fluids  act  upon  proteids,  carbohydrates, 
and  fats;  in  them  are  special  chemical  compounds,  called  fer- 
ments or  enzymes,  which  break  up  complex  molecules  without 
themselves  becoming  changed  chemically.  The  three  most 
important  enzymes  are  (i)  trypsin,  which  dissolves  proteid, 

(2)  diastase,  which  breaks  up  molecules  of  carbohydrates,  and 

(3)  steapsin,  which  acts  upon  fats.     These  three  enzymes  are 


THE  EARTHWORM   AND  ANNELIDS  IN   GENERAL       171 

probably  present  in  the  digestive  fluids  of  the  earthworm.  The 
proteids  are  changed  into  peptones,  the  carbohydrates  into  a 
sugar  compound,  and  the  fats  are  divided  into  glycerin  and  fatty 
acids. 

ABSORPTION.  —  The  food  is  now  ready  for  absorption.  This  is 
accomplished  through  the  wall  of  the  intestine  by  a  process 
known  as  osmosis,  assisted  by  an  ameboid  activity  of  some  of 
the  epithelial  cells.  Osmosis  is  the  passage  of  a  liquid  through 
a  membrane. 

CIRCULATION.  —  Upon  reaching  the  blood,  the  absorbed  food  is 
carried  to  various  parts  of  the  body  by  circulation,  the  details  of 
which  are  described  in  another  place  (pp.  174-175).  Absorbed 
food  also  makes  its  way  into  the  ccelomic  cavity  and  is  carried 
directly  to  those  tissues  bathed  by  the  ccelomic  fluid.  In  one- 
celled  animals,  and  in  such  Metazoons  as  Hydra,  Planaria,  and 
Ascaris  no  circulatory  system  is  necessary,  since  the  food  either  is 
digested  within  the  cells  or  comes  into  direct  contact  with  them; 
but  in  large,  complex  animals  a  special  system  of  organs  must  be 
provided  to  enable  the  proper  distribution  of  nutriment. 

ASSIMILATION,  as  in  the  types  already  described,  is  the  addition 
of  new  particles  among  the  preexisting  particles  of  protoplasm. 

Vascular  System.  —  The  blood  of  the  earthworm  is  contained 
in  a  complicated  system  of  tubes  which  ramify  to  all  parts  of  the 
body.  A  number  of  these  tubes  are  large  and  centrally  located; 
these  give  off  branches  which  likewise  branch,  finally  ending  in 
exceedingly  thin  tubules,  the  capillaries.  The  functions  of  this 
system  of  tubes  are  to  carry  nourishment  from  the  alimentary 
canal  to  all  parts  of  the  body,  to  transport  waste  products,  and  to 
convey  the  blood  to  a  point  near  the  surface  of  the  body  where 
oxygen  may  be  obtained  and  supplied  to  the  tissues. 

BLOOD.  —  The  blood  of  the  earthworm  consists  of  a  plasma  in 
which  are  suspended  a  great  number  of  colorless  cells,  called 
corpuscles.  Its  red  color  is  due  to  a  pigment  termed  hcemoglobin, 
which  is  dissolved  in  the  plasma.  In  vertebrates  the  haemoglobin 
is  located  in  the  blood  corpuscles. 


172 


AN  INTRODUCTION  TO  ZOOLOGY 


BLOOD  VESSELS.  —  Following  a  custom,  which  is  so  firmly 
established  as  to  make  its  abandonment  inadvisable,  we  shall  call 
the  blood  tubes  by  the  inappropriate  name  "  vessels."  Longi- 


FIG.  88.  Diagrams  showing  the  arrangement  of  the  blood  vessels  in  the  earth- 
worm. A,  longitudinal  view  of  the  vessels  in  somites  VIII,  IX,  and 
X ;  B,  transverse  section  of  same  region ;  C,  longitudinal  view  of  the 
vessels  in  the  intestinal  region ;  D,  transverse  section  through  the 
intestinal  region  ;  a/.  *.,  afferent  intestinal  vessel ;  c.v.,  parietal  vessel; 
ef.  /.,  efferent  intestinal  vessel ;  ht.,  heart ;  i,  intestine ;  it.,  intestino- 
tegumentary  ;  «/.,  lateral-neural  vessel ;  ces.,  oesophagus  ;  s.,  septa  ;  sb., 
ventral  vessel ;  sn.,  sub-neural  vessel ;  sp.,  dorsal  vessel ;  ty.,  typhlosolar 
vessel.  (From  Bourne  after  Benham.) 


THE   EARTHWORM   AND  ANNELIDS  IN   GENERAL       173 

tudinal  vessels,  five  in  number,  extend  from  the  anterior  to  the 
posterior  end  of  the  body;  these  are  connected  with  one  another 
and  with  various  organs  by  branches  more  or  less  regularly 
arranged. 

(1)  The  largest  and  most  important  of  the  longitudinal  trunks 
is  the  dorsal  or  supra-intestinal  (Fig.  88,  sp.),  which  runs  along 
the  dorsal  surface  of  the  alimentary  canal,  from  the  posterior 
end  of  the  body  to  the  pharynx. 

(2)  Just  beneath  the  alimentary  canal  lies  the  ventral  or  sub- 
intestinal  trunk  (sb.).     This  likewise  extends  from  the  posterior 
end  of  the  body  to  the  pharynx,  where  it  divides  into  many  small 
branches. 

(3)  The  sub-neural  trunk  (sn.)  traverses  the  entire  length  of  the 
body  on  the  under  side  of  the  ventral  nerve  cord. 

(4  and  5)  The  lateral-neural  trunks  (nl.)  lie  one  on  either  side 
of  the  ventral  nerve  cord;  they  are  smaller  than  the  other  longi- 
tudinal trunks. 

The  branches  from  the  longitudinal  trunks  in  the  anterior  part 
of  the  body  differ  from  those  in  the  region  of  the  intestine.  A 
pair  of  short  thick  tubes  connect  the  dorsal  with  the  ventral 
vessels  in  each  of  the  five  segments  from  VII  to  XI.  These  are 
known  as  hearts  (tit.)  because  of  their  power  of  contractility. 
Small  branches  from  the  hearts  supply  the  septa  immediately 
posterior  to  them  (Fig.  88,  A).  In  somite  X,  two  intestino- 
tegumentary  vessels  (Fig.  88,  it.  in  A  and  B)  arise,  one  on  either 
side  of  the  dorsal  trunk.  Each  extends  anteriorly,  sending  a  pair 
of  branches  to  the  oesophagus  in  every  segment  from  X  to  VI, 
and  receiving  branches  from  the  integument  and  nephridia  in  the 
same  somites.  The  ventral  trunk  gives  rise  in  each  segment  to 
two  vessels,  one  going  to  the  nephridium,  the  other  to  the  body 
wall  (Fig.  88,  D).  The  sub-neural  receives  vessels  from  the  in- 
tegument. 

In  the  region  of  the  intestine  the  dorsal  is  connected  with  the 
sub-neural  trunk  in  each  segment  by  a  pair  of  parietal  vessels 
(Fig.  88,  co.  in  C  and  D).  The  latter  receive  branches  from  the 


174  AN   INTRODUCTION  TO   ZOOLOGY 

nephridia  and  body  wall  (D).  The  dorsal  trunk  also  communi- 
cates with  the  intestine  by  means  of  branches.  Two  pairs  of 
vessels  pass  from  it  to  the  walls  of  the  intestine  in  every  segment 
(ef.  i.  in  C).  The  blood  is  collected  from  the  intestine  by  two  pairs 
of  vessels  which  enter  a  longitudinal  typhlosolar  tube  (ty.  in  C). 
The  latter  is  connected  with  the  dorsal  trunk  by  three  or  four 
short  tubes  in  each  somite.  The  ventral  trunk  in  the  intestinal 
region  gives  off  in  every  segment  a  pair  of  vessels  each  of  which 
divides,  sending  a  branch  to  the  nephridium  and  one  to  the  body 
wall  (sb.  in  D). 

STRUCTURE  OF  BLOOD  VESSELS. — The  dorsal  trunk  and  hearts 
determine  the  direction  of  the  blood  flow,  since  they  furnish  the 
power  by  means  of  their  muscular  walls.  The  wall  of  the  dorsal 
trunk  is  composed  of  four  layers:  (i)  an  inner  epithelium  of  thin 
cells,  (2)  a  connective  tissue  layer;  (3)  a  well-developed  stratum 
of  circular  muscle  fibers,  and  (4)  an  outer  covering  of  chlorogogen 
cells.  The  walls  of  the  hearts  are  similar  in  structure.  Pairs  of 
forwardly  directed  valves  are  situated  in  the  dorsal  trunk  just 
behind  the  openings  of  the  parietal  vessels.  The  valves  do  not 
prevent  the  flow  of  the  blood  in  an  anterior  direction,  but  the 
dorsal  trunk,  when  constricted,  is  completely  closed  by  them, 
making  the  backward  flow  impossible.  Pairs  of  valves  are  also 
present  in  the  dorsal  trunk  just  in  front  of  the  openings  of  the 
hearts.  Other  valves  occur  in  the  vessels  directly  connected  with 
the  dorsal  trunk. 

The  ventral  trunk  does  not  possess  a  layer  of  circular  muscle 
fibers,  and,  therefore,  has  not  the  power  to  contract.  It  is 
strengthened  by  a  thick  layer  of  fibrous  connective  tissue.  The 
sub-neural  and  lateral-neural  trunks,  as  well  as  all  the  smaller 
branches,  have  simply  an  inner  epithelium  and  an  outer  connec- 
tive tissue  layer  (146). 

CIRCULATION.  —  The  flow  of  blood  in  the  blood  vessels  is  not 
segmental,  but  systemic.  Blood  is  forced  forward  by  wave-like 
contractions  of  the  dorsal  trunk,  beginning  at  the  posterior  end 
and  traveling  quickly  anteriorly.  These  contractions  are  said 


THE  EARTHWORM  AND  ANNELIDS  IN  GENERAL     175 

to  be  peristaltic,  and  have  been  likened  to  the  action  of  the  fingers 
in  the  operation  of  milking.  The  valves  in  the  walls  of  the  dorsal 
trunk  prevent  the  return  of  blood  from  the  anterior  end.  In 
somites  VII  to  XI  the  blood  passes  from  the  dorsal  trunk  into 
the  hearts,  and  is  forced  by  them  both  forward  and  backward 
in  the  ventral  trunk.  The  valves  in  the  heart  also  prevent  the 
backward  flow.  From  the  ventral  trunk  the  blood  passes  to  the 
body  wall  and  nephridia.  Blood  is  returned  from  the  body  wall 
to  the  lateral-neural  trunks.  The  flow  in  the  sub-neural  trunk  is 
toward  the  posterior  end,  then  upward  through  the  parietal 
vessels  into  the  dorsal  trunk.  The  anterior  region  receives  blood 
from  the  dorsal  and  ventral  trunks.  The  blood  which  is  carried 
to  the  body  wall  and  integument  receives  oxygen  through  the 
cuticle,  and  is  then  returned  to  the  dorsal  trunk  by  way  of  the 
sub-neural  trunk  and  the  intestinal  connectives.  Because  of  its 
proximity  to  the  sub-neural  trunk,  the  nervous  system  receives  a 
continuous  supply  of  the  freshest  blood  (143,  146). 

Respiration.  —  The  earthworm  possesses  no  respiratory  sys- 
tem, but  obtains  oxygen  and  gets  rid  of  carbon  dioxide  through 
the  moist  outer  membrane.  Many  capillaries  lie  just  beneath  the 
cuticle,  making  the  exchange  of  gases  easy.  The  oxygen  is  com- 
bined with  the  haemoglobin. 

Excretory  Organs.  —  Most  of  the  excretory  matter  is  carried 
outside  of  the  body  by  a  number  of  coiled  tubes,  termed  nephridia 
(Fig.  85,  neph.),  a  pair  of  which  are  present  in  every  somite  except 
the  first  three  and  the  last.  A  nephridium  occupies  part  of  two 
successive  somites;  in  one  is  a  ciliated  funnel,  the  nephrostome 
(Fig.  85,  nephrost.),  which  is  connected  by  a  thin  ciliated  tube 
with  the  major  portion  of  the  structure  in  the  somite  posterior 
to  it.  Three  loops  make  up  the  coiled  portion  of  the  nephridium. 
The  thin  tube  mentioned  above  is  a  single  row  of  hollow  cylin- 
drical cells  placed  end  to  end;  it  extends  through  one  loop  and  a 
half,  and  connects  with  a  larger  tube  brown  in  color  and  ciliated 
throughout  its  central  cavity.  This  portion,  known  as  the  middle 
tube,  opens  into  a  third  tube,  which  is  wider,  but  without  cilia. 


i76 


AN  INTRODUCTION  TO  ZOOLOGY 


Leading  from  the  third  tube  to  the  external  aperture  is  a  large 
muscular  duct.  The  structure  of  the  nephrostome  and  of  the 
thin  tube  just  behind  it  is  of  particular  interest.  The  nephro- 
stome (Fig.  89)  consists  of  a  large  crescentic  central  cell  (c.c.)  sur- 
rounded by  a  layer  of 
ciliated  marginal  cells 
(cp.) ;  these  join  a  num- 
ber of  grooved  cells  (cf.) 
which  lead  to  the  thin 
ciliated  tube.  The  cen- 
cf-  tral  cavity  of  this  cil- 
iated tube  is  not 
.~cp.  surrounded  by  cells, 
that  is,  intercellular, 
but  is  intracellular,  pass- 
ing directly  through  the 
cells.  The  tube  thus 
resembles  a  drainpipe, 
the  sections  of  which 
are  represented  by  sin- 
gle cells  placed  end  to 
end. 

FIG.  89.  Nephrostome  of  the  earthworm,  c.c.,  EXCRETION.  — A  ne- 
central  cell ;  cf.,  grooved  cells  ;  cp.,  ciliated  phridium  functions  in 
marginal  cells.  (From  Dahlgren  and  the  flowing  manner. 

Kepnen)  The  cilia  on  the  neph- 

rostome and  in  the  thin  tube  and  middle  tube  create  a  current 
through  the  muscular  duct  which  leads  to  the  exterior.  Solid 
waste  particles  which  may  be  floating  about  in  the  coelomic  fluid 
are  drawn  into  the  nephridium  by  this  current,  and  pass  to  the 
outside.  Waste  matter  in  solution  is  taken  from  the  blood  by 
the  glands  of  the  "  wide  tube  "  and  stored  in  the  large  sac-like 
muscular  duct  until  excreted. 

Nervous  System.  —  The  nervous  system  differs  from  that  of 
Planaria  and  Ascaris  in  being  more  concentrated.     At  the  an- 


\ 


THE  EARTHWORM   AND   ANNELIDS  IN   GENERAL     177 

terior  end  of  the  body  in  somite  III  is  a  bi-lobed  mass  of  nervous 
tissue  (Fig.  90,  2);  this  is  called  the  brain,  or  supra-pharyngeal 
ganglion,  because  of  its  position  on  the  dorsal  surface  of  the 
pharynx.  Two  large  nerve  cords,  the  circum-pharyngeal  connec- 
tives (j),  pass  around  the  pharynx  one  on  either  side,  connecting 
the  brain  with  a  pair  of  ganglia  lying  beneath  the  pharynx  in 


FIG.  90.  Diagram  of  the  anterior  end  of  an  earthworm  to  show  the  arrange- 
ment of  the  nervous  system.  /,  prostomium ;  2,  brain ;  j,  circum- 
pharyngeal  connective;  4,  sub-pharyngeal  ganglion;  5,  mouth;  6, 
pharynx ;  7,  setae ;  8,  tactile  nerves  to  prostomium ;  p,  dorsal  nerves ; 
10,  ventral  nerves.  (From  Shipley  and  MacBride.) 

somite  IV.  These  are  the  sub-pharyngeal  ganglia  (4).  Every 
somite  posterior  to  IV  contains  a  ganglionic  mass  which  is  con- 
nected by  a  nerve  cord  with  the  ganglion  in  the  preceding  somite, 
and  also,  with  the  exception  of  the  last,  with  that  in  the  succeed- 
ing somite.  This  connected  row  of  ganglia  is  called  the  ventral 
nerve  cord,  and,  together  with  the  supra-pharyngeal  ganglia,  con- 
stitutes the  central  nervous  system.  The  nerves  which  pass  from 
the  central  nervous  system  to  all  parts  of  the  body,  and  which 
pass  to  it  from  the  body  wall  and  internal  organs,  constitute  the 

,    N 


178  AN  INTRODUCTION  TO  ZOOLOGY 

peripheral  nervous  system  (Fig.  90,  Q  and  10).  The  supra-pharyn- 
geal  ganglia  supply  the  prostomium  with  two  large  nerves  which 
give  off  many  branches  (8);  they  also  send  nerves  into  somites 
II  and  III.  One  nerve  extends  out  from  each  circum-pharyngeal 
connective.  In  each  somite  from  IV  to  the  posterior  end  of  the 
body,  three  pairs  of  nerves  arise,  two  pairs  from  the  ganglionic 
mass  and  one  pair  from  the  sides  of  the  nerve  cord  just  behind 
the  septum  which  separates  the  somite  from  the  one  preceding. 

Each  enlargement  of  the  ventral  nerve  cord  really  consists  of 
two  ganglia  which  are  closely  fused  together.  In  transverse 
section  these  fused  ganglia  are  seen  to  be  surrounded  by  an  outer 
thin  layer  of  epithelium,  the  peritoneum,  and  an  inner  muscular 
sheath  containing  blood  vessels  and  connective  tissue  as  well  as 
muscle  fibers.  Near  the  dorsal  surface  are  three  large  areas, 
each  surrounded  by  a  thick  double  sheath  and  containing  a  bundle 
of  nerve  fibers.  These  are  called  neurochords  or  " giant  fibers" 
Large  pear-shaped  nerve  cells  are  visible  near  the  periphery  in  the 
lateral  and  ventral  parts  of  the  ganglion. 

The  nerves  of  the  peripheral  nervous  system  are  either  efferent 
or  afferent.  Efferent  nerve  fibers  are  extensions  from  cells  in  the 
ganglia  of  the  central  nervous  system.  They  pass  out  to  the 
muscles  or  other  organs,  and,  since  impulses  sent  along  them  give 
rise  to  movements,  the  cells  of  which  they  are  a  part  are  said  to  be 
motor  nerve  cells.  The  afferent  fibers  originate  from  nerve  cells 
in  the  epidermis  which  are  sensory  in  function,  and  extend  into 
the  ventral  nerve  cord. 

The  functions  of  nervous  tissue  are  perception,  conduction,  and 
stimulation  (89).  These  are  usually  performed  by  nerve  cells, 
called  neurons.  The  neuron  theory  "  supposes  that  there  is  no 
nerve  fiber  independent  of  nerve  cell  and  that  the  cell  with 
all  its  prolongations  is  a  unit  or  a  neuron;  that  these  units  are 
not  united  to  one  another  anatomically,  but  act  together  physio- 
logically by  contact;  that  the  entire  nervous  system  consists  of 
superimposed  neurons;  .  .  ."  (136,  p.  633). 

The  reflex  carried  out  either  consciously  or  unconsciously  is 


THE  EARTHWORM  AND  ANNELIDS  IN  GENERAL    179 

considered  the  physiological  unit  of  nervous  activity.  The  ap- 
paratus required  for  a  simple  reflex  in  the  body  of  an  earthworm 
is  represented  in  Figure  91.  A  primary  sensory  neuron  (sc.), 
lying  at  the  surface  of  the  body,  sends  a  fiber  (sf.)  into  the  ventral 
nerve  cord  where  it  branches  out;  these  branches  are  in  physio- 
logical continuity  with  branches  from  a  primary  motor  neuron 
(me. )  lying  in  the  ganglion  of  the  ventral  nerve  cord.  The  second 


FIG.  91.  Transverse  section  of  the  ventral  nerve  chain  and  surrounding 
structures  of  an  earthworm,  cm.,  circular  muscles ;  ep.,  epidermis ; 
1m.,  longitudinal  muscles;  me.,  motor  cell  body;  mf.,  motor  nerve 
fiber;  sc.,  sensory  cell  body;  sf.,  sensory  nerve  fiber;  vg.,  ventral  gan- 
glion. (From  Parker  in  Pop.  Sci.  Monthly,  modified  after  Retzius.) 

neuron  (me.)  sends  fibers  (mf.)  into  a  reacting  organ,  which  in  this 
case  is  a  muscle.  These  fibers  extending  to  the  reacting  organ  are 
called  motor  fibers  (mf.) ;  those  leading  to  the  ventral  nerve  cord 
are  termed  sensory  fibers  (sf.).  The  first  neuron,  or  receptor,  re- 
ceives the  stimulus  and  produces  the  nerve  impulse;  the  second 
neuron,  the  adjuster,  receives,  directs,  and  modifies  the  impulse; 
and  the  muscle  or  other  organ  stimulated  to  activity  is  the 
effector.  Within  the  ventral  nerve  cord  are  association  neurons 
whose  fibers  serve  to  connect  structures  within  one  ganglion  or  two 
succeeding  ganglia.  These  short  neurons  overlap  one  another, 
and  are  doubtless  responsible  for  the  muscular  waves  which  pass 
from  the  anterior  to  the  posterior  end  of  the  worm  during  loco- 
motion. The  three  giant  fibers,  which  lie  in  the  dorsal  part  of 


i8o 


AN   INTRODUCTION  TO  ZOOLOGY 


per. 


c.- 


the  ventral  nerve  cord  throughout  almost  its  entire  length,  are 
connected  by  means  of  fibrils  with  nerve  cells  in  the  ganglia 
and  probably  distribute  the  impulse  that  causes  a  worm  to  con- 
tract its  entire  body  when  strongly 
stimulated  (151). 

Sense  Organs.  —  The  sensitive- 
ness of  Lumbricus  to  light  and 
other  stimuli  is  due  to  the  presence 
of  a  great  number  of  epidermal 
sense  organs.  These  are  groups  of 
sense  cells  (Fig.  92,  sn.  c.)  connected 
with  the  central  nervous  system  by 
means  of  nerve  fibers  (nv.),  and 
communicating  with  the  outside 
world  through  sense  hairs  (per.) 
which  penetrate  the  cuticle.  More 
of  these  sense  organs  occur  at  the 
anterior  and  posterior  ends  than  in 
any  other  region  of  the  body  (see 
Fig.  90,  8).  The  epidermis  of  the 
earthworm  is  also  supplied  with 
efferent  nerve  fibers  which  penetrate 
between  the  epidermal  cells  forming 
a  sub-epidermal  network  1,144,  147)- 
Reproduction.  -  -  FEMALE  OR- 
GANS (Fig.  93). — The  female  re- 
FIG.  02.  Tactile  nerve  endings  in  ,  .  ,. 

the  integument  of  the  earth-    Productive   organs   are   a   pair   of 
*orm.     per.,   sensory    hairs    ovaries  (0)  in  somite  XIII,  two  ovi- 
projecting  through  the  cuti-    ducts  (OD)  in  somites  XIII  and  XIV, 
:le;  nv.,  nerve  ;   sn.  c.,  sense    ari(l  two  pairs  of  seminal  receptacles 
(S)  or  spermathecce  lying  in  somites 
IX  and  X.     The  ovaries  are  small 
pear-shaped  bodies.     They  lie  one  on  either  side  of   the   mid- 
ventral  line  in  somite  XIII,  and  are  attached  by  their  larger 
ends  to  the  ventral  part  of  the  anterior  septum.     The  oviducts 


cells.     (From  Dahlgren  and 
Kepner.) 


THE  EARTHWORM   AND  ANNELIDS  IN   GENERAL      i8l 

consist  of  the  following  parts:  posterior  to  each  ovary  is  a 
ciliated  funnel  which  passes  through  the  septum  between  somites 
XIII  and  XIV,  enlarges  into  an  egg  sac  (R),  and  then  narrows 


FIG.  93.  Diagram  of  the  reproductive  organs  of  the  earthworm,  dorsal  view. 
A,  B,  C,  seminal  vesicles ;  N,  nerve  cord ;  O,  ovary ;  OD,  oviduct ;  R, 
egg  sac  ;  S,  spermatheca ;  SF,  seminal  funnel ;  T,  testes ;  VD,  vas  defer- 
ens.  (From  Marshall  and  Hurst.) 

into  a  thin  duct  (OD)  which  opens  to  the  outside  on  the  ventral 
surface  of  the  body  near  the  center  of  somite  XIV.  The  sper- 
mathecae  are  white  globular  sacs  situated  near  the  ventral  body 


l82  AN  INTRODUCTION  TO  ZOOLOGY 

wall,  one  pair  in  somite  IX,  the  other  in  somite  X.  They  open 
to  the  outside  through  the  spermathecal  pores  between  somites 
IX  and  X,  and  X  and  XI. 

MALE  ORGANS  (Fig.  93).  — The  male  reproductive  organs  are 
two  pairs  of  glove-shaped  testes  (T),  one  pair  in  somite  X,  the  other 
in  somite  XI.  They  occupy  positions  in  the  somites  similar  to 
that  of  the  ovaries.  Behind  each  testis  is  a  ciliated  funnel  (SF) 
shaped  like  a  rosette;  this  is  the  opening  of  the  sperm  duct  or 
vas  deferens  (VD).  This  duct  passes  through  the  septum  just 
back  of  the  funnel,  forms  several  convolutions,  and  then  extends 
backward  near  the  ventral  surface.  The  two  sperm  ducts  arising 
on  either  side  of  the  mid- ventral  line  unite  in  somite  XII  and  then 
run  back  as  a  single  tube,  opening  to  the  outside  through  the 
spermiducal  pore  on  somite  XV.  In  a  sexually  mature  earth- 
worm, the  testes  and  funnel-shaped  inner  openings  of  the  sperm 
ducts  are  inclosed  by  large  white  sacs,  the  seminal  vesicles  (A, 
B,  C),  lying  in  somites  IX  to  XII.  There  are  three  pairs  of 
these  sperm  sacs,  one  in  somite  IX  (A),  one  in  somite  XI  (C), 
and  the  third  in  somite  XII.  In  somites  X  and  XI  are  central 
reservoirs  (B). 

COPULATION.  —  Many  of  the  events  which  precede  the  laying  of 
eggs  by  the  earthworm  have  not  yet  been  learned.  Reproduc- 
tion is  best  known  in  the  common  manure  worm,  Allolobophora 
fcetida.  The  breeding  season  begins  early  in  the  spring  and  con- 
tinues until  late  in  the  autumn.  Sexually  mature  worms  possess 
a  much  swollen  clitellar  region.  Although  both  eggs  and  sperms 
are  produced  by  every  individual,  self-fertilization  does  not  take 
place,  but  the  eggs  of  one  worm  are  fertilized  by  the  sperm  of 
another.  The  sperm  are  transferred  from  one  worm  to  another 
during  a  process  called  copulation,  which  usually  takes  place 
just  beneath  the  surface  of  the  earth.  Two  worms  come  together 
with  their  heads  in  opposite  directions,  and  their  ventral  surfaces 
opposed,  as  shown  in  Figure  94,  A.  Each  secretes  about  itself 
a  tube  of  slime  which  extends  from  about  the  eighth  to  the  thirty- 
sixth  somite.  Four  bandlike  thickenings,  probably  of  the  slime 


THE  EARTHWORM   AND   ANNELIDS  IN   GENERAL 


183 


tubes,  encircle  both  worms  at  the  anterior  and  posterior  edges  of 
the  clitellar  regions.  These  hold  the  worms  firmly  together  and 
may  later  aid  in  closing  the  ends  of  the  cocoons.  The  slime  tube 
protects  the  co- 
coon as  it  devel- 
ops, and  confines 
the  seminal  fluid 
and  spermato- 
phores.  During 

copulation  sperms  are  not  exchanged  between 
the  spermathecae  of  the  two  worms,  but  the 
spermathecae  of  each  are  loaded  from  the 
spermaducal  pore  of  the  other,  or  spermato- 
phores  formed  in  the  spermathecae  of  one  in- 
dividual are  attached  to  the  skin  of  the  other. 
COCOONS.  —  Cocoons  are  formed  either 
during  copulation,  or  after  the  two  worms 
have  separated.  In  the  former  case  a  band 
is  secreted  about  the  clitellar  region  of  one 
worm  and  three  or  more  opposite  somites  of 
the  other  worm.  At  first  the  cocoons  are 
perfectly  white  (Fig.  94,  B),  but  after  deposi-  FlG-  94-  A,  the  an- 
tion  the  exposure  to  the  air  changes  them  to 
a  yellow  color.  When  copulation  is  completed, 
the  worms  slowly  withdraw  backward  from  the 
slime  tube  and  cocoon.  As  the  cocoon  is 
slipped  over  the  head,  its  ends  contract, 
forming  a  turnip-shaped  capsule,  lying  within 
the  slime  tube  (Fig.  94,  B).  The  cocoon 
after  deposition  contains,  on  an  average,  four 
eggs,  each  of  which  has  been  penetrated  by 
from  one  to  as  many  as  nine  spermatozoa. 
The  nucleus  of  a  single  spermatozoon  unites 
with  the  egg  nucleus;  the  remaining  sperma- 
tozoa disintegrate  (141,  142). 


tenor  segments 
of  two  copulat- 
ing earthworms. 
Slime  tubes  en- 
circle the  pair 
from  the  8th  to 
the  33d  seg- 
ment. B,  co- 
coon, freshly 
deposited,  of  an 
earthworm  sur- 
rounded by  one 
half  of  a  slime 
tube.  (After 
Foot  in  Journ- 
Morph.) 


184  AN  INTRODUCTION   TO  ZOOLOGY 

Before  considering  the  embryology  of  the  earthworm,  the 
development  of  the  eggs  and  spermatozoa  and  the  history  of  these 
previous  to  fertilization  must  be  described. 

SPERMATOGENESIS.  —  During  the  development  of  the  spermato- 
zoa, the  primordial  germ  cells  separate  from  the  testes,  and  lie 
in  the  cavities  of  the  seminal  vesicles.  The  nucleus  of  each  germ 
cell  divides  into  2,  4,  8,  or  16  daughter  nuclei,  which  become 
arranged  in  a  single  layer  near  the  surface  of  the  cell.  Cell  walls 
now  appear,  extending  inward  from  the  periphery,  the  result  being 
a  colony  of  cells  attached  by  cytoplasmic  pedestals  to  a  central 
non-nucleated  mass.  The  cells  of  the  colony  then  divide,  increas- 
ing the  number  to  32.  64,  128  or  more.  A  dissociation  of  the 
colony  into  several  parts  now  takes  place,  each  part  containing 
a  number  of  spermatogonia.  Such  a  spermatogonial  group  finally 
becomes  a  spherical  morula  of  32  primary  spermatocytes,  which 
are  still  fastened  by  cytoplasmic  threads  to  a  central  body  called 
the  blastophore.  Each  group  of  primary  spermatocytes  gives 
rise  to  64  secondary  spermatocytes,  and  these  divide  to  form  128 
spermatids.  The  latter  then  metamorphose  into  spermatozoa. 
The  number  of  chromosomes  in  the  spermatozoa  is  sixteen;  this 
is  one  half  the  number  contained  in  the  somatic  cells,  the  reduc- 
tion having  taken  place  during  maturation  by  the  union  of  the 
chromosomes  two  by  two  in  the  secondary  spermatocytes,  and  a 
subsequent  separation  when  the  spermatids  were  formed  (138, 
140). 

During  the  later  stages  of  copulation  the  spermatozoa  are 
drawn  into  the  rosette-like  funnels  (Fig.  93,  SF)  of  the  vas  adeje- 
rentia,  pass  through  these  ducts,  and  are  transferred  to  the 
spermathecae  of  the  other  worm.  Here  they  are  stored  until  the 
cocoon  passes  over  the  openings  of  the  spermathecae  during  the 
withdrawal  of  the  worm,  when  some  of  them  pass  into  the  nutri- 
ent fluid  which  has  been  secreted  into  the  cavity  of  the  cocoon. 

OOGENESIS. — The  eggs  develop  in  the  ovaries  (Fig.  93,  O). 
These  are  pear-shaped  bodies  composed  of  egg  cells  in  various 
stages  of  growth.  The  basal  portion  of  each  ovary  consists  of 


FIG.  95.  Stages  in  the  embryology  of  the  earthworm.  A,  two-celled  stage ; 
B,  four-celled  stage  ;  C,  six-celled  stage  ;  D,  eight-celled  stage  ;  E,  bias- 
tula  in  section;  F,  blastula  flattened;  G,  stage  showing  mesoblastic 
bands  ;  H,  gastrula  ;  J,  older  gastrula  ;  K,  longitudinal  section  of  later 
stage,  bl.,  blastoccel ;  bp.,  blastopore  ;  ec.,  ectoderm  ;  en.,  entoderm  ; 
mac.,  macromere  ;  mes.,  mesoderm  ;  mic.,  micromere  ;  mm.,  mesomeres. 
(From  Bourne  after  Wilson.) 


THE  EARTHWORM   AND  ANNELIDS  IN   GENERAL     185 

primitive  germ  cells;  from  here  to  the  pointed  end  of  the  ovary 
the  cells  increase  in  size,  those  toward  the  extreme  end  being 
recognizable  as  eggs,  each  of  which  is  surrounded  by  a  follicle 
of  small  nutritive  cells.  Eggs  separate  from  the  end  of  the  ovaries 
and  pass  into  the  body  cavity.  From  here  they  enter  the  ciliated 
funnel  of  the  oviduct,  thence  into  the  egg  sac  (Fig.  93,  R),  where 
part  of  the  maturation  processes  occur.  From  here  they  either 
pass  out  into  the  cavity  of  the  slime  tube  and  are  conveyed  from 
the  external  openings  of  the  oviduct  on  somite  XIV  to  the  co- 
coon, or  enter  the  cocoon  when  it  passes  over  this  somite  during 
deposition  (142). 

FERTILIZATION.  —  The  eggs  are  penetrated  by  spermatozoa 
after  the  cocoon  is  shed.  At  this  time  they  have  not  completed 
their  maturation  processes  (141,  142). 

Embryology.  —  The  eggs  of  the  earthworm  are  holoblastic, 
but  cleavage  is  unequal,  the  first  division  resulting  in  one  large 
and  one  small  cell  (Fig.  95,  A).  The  second  cleavage  divides 
the  small  cell  into  two  equal  parts  and  cuts  off  a  small  cell  from  the 
larger  (B).  Because  of  their  difference  in  size,  the  large  cells  are 
called  macromeres  (mac.)  and  the  small  cells,  micromeres  (mic.). 
Cleavage  becomes  irregular  after  the  second  division.  Both  the 
macromeres  and  micromeres  continue  to  divide,  the  number  of 
cells  increasing  to  six  (C),  eight  (D),  etc.  Soon  a  cavity,  the 
blastoccel  (bl.),  appears  between  the  micromeres  and  macromeres, 
and  a  hollow  sphere  recognizable  as  a  blastula  results  (E).  Two 
of  the  larger  cells  of  the  blastula  lying  side  by  side  behave  differ- 
ently from  the  others.  They  project  down  into  the  blastoccel, 
and,  by  repeated  divisions,  give  rise  to  two  rows  of  small  cells 
(mes.  and  mm.  in  F  and  G).  Because  of  the  fact  that  these  two 
cells  give  rise  to  the  mesoderm,  they  are  termed  mesomeres,  and 
the  two  rows  of  cells  derived  from  them,  the  mesoblastic  bands. 
While  the  mesomeres  are  dividing,  the  blastula  becomes  flat- 
tened (F),  the  larger  cells  form  a  plate  of  clear  columnar  cells 
(mac.),  and  the  small  cells  spread  out  into  a  thin  dome-shaped 
epithelium  (mic.). 


l86  AN  INTRODUCTION  TO  ZOOLOGY 

Bilateral  symmetry  is  already  established  at  this  early  period: 
the  mesoblastic  bands  lie  along  what  will  become  the  longitudinal 
axis  of  the  future  worm,  and  the  mesomeres  occupy  the  posterior 
end.  A  gastmla  is  now  formed  by  the  invagination  of  the  plate 
of  large  cells  (H) ;  the  edges  of  the  cavity  thus  produced  fold  in 
until  only  a  slit  remains  (J).  This  slit  is  the  blastopore  (bp.), 
and  the  cavity  is  the  enteron,  which  later  becomes  part  of  the 
alimentary  canal.  Soon  the  slit  closes,  except  at  one  end  where  a 
pore,  the  future  mouth,  remains.  The  three  germ  layers  are  at 
this  time  quite  distinct,  and  are  well  shown  in  a  longitudinal 
section  of  the  gastrula  (K).  The  large  clear  cells  which  invagi- 
nated  line  the  enteron,  becoming  the  entoderm  (en.)]  the  dome- 
shaped  epithelium  of  small  cells  covers  the  outer  surface  and  repre- 
sents the  ectoderm  (ec.) ;  and  between  these  two  layers  are  the  two 
rows  of  mesoblastic  bands  which  give  rise  to  the  mesoderm  (mes.) 

(157)- 

The  detailed  history  of  these  germ  layers  is  too  long  and  com- 
plicated to  be  discussed  in  a  book  of  this  character.  The  develop- 
ment of  the  mesodermal  layers  and  ccelom  should,  however,  be 
mentioned.  The  mesoderm  becomes  separated  into  two  layers 
between  which  a  cavity,  the  ccelom,  is  formed.  The  outer  layer, 
called  the  somatopleure,  clings  to  the  ectoderm,  and  gives  rise  to 
the  muscles  of  the  body  wall;  the  inner  layer,  called  the  splanch- 
nopleure,  remains  attached  to  the  enteron  and  gives  rise  to  the 
muscles  of  the  alimentary  canal.  All  other  structures  of  meso- 
dermal origin  are  derived  from  these  two  layers.  After  the  es- 
tablishment of  the  germ  layers  as  described  above,  the  embryo 
elongates,  and  finally  becomes  vermiform,  escaping  from  the 
cocoon  in  about  two  or  three  weeks. 

Behavior.  —  EXTERNAL  STIMULI.  —  The  external  stimuli  that 
have  been  most  frequently  employed  in  studying  the  behavior  of 
earthworms  are  those  dealing  with  thigmotropism,  chemotropism, 
and  phototropism  (135,  145,  150,  152,  153,  155,  156). 

THIGMOTROPISM.  —  Mechanical  stimulation,  if  continuous  and 
not  too  strong,  calls  forth  a  positive  reaction;  the  worms  live 


THE   EARTHWORM   AND   ANNELIDS  IN   GENERAL        187 

where  their  bodies  come  in  contact  with  solid  objects;  they 
apparently  like  to  feel  the  walls  of  their  burrows  against  their 
bodies  or,  when  outside  of  their  burrows,  to  lie  or  crawl  upon  the 
ground  (155).  Reactions  to  sounds  are  not  due  to  the  presence 
of  a  sense  of  hearing,  but  to  the  contact  stimuli  produced  by  vi- 
brations. Darwin  showed  that  musical  tones  produced  no  re- 
sponse, but  that  the  worms  contained  in  a  flower  pot  drew  back 
into  their  burrows  immediately  when  a  note  was  struck,  if  the 
pot  were  placed  upon  a  piano,  this  result  being  due  to  vibrations. 

CHEMOTROPISM.  —  Contact  is  not  sufficient  to  cause  burrowing, 
but  a  combination  of  mechanical  and  chemical  stimuli  seems 
necessary  —  at  least  this  is  true  of  the  small  earthworm,  Allolo- 
bophora  fceiida,  found  in  heaps  of  manure.  A  worm  of  this 
species  does  not  burrow  when  placed  in  contact  with  dry  filter 
paper;  but  immediately  responds  if  the  paper  is  wet  with  water 
or  liquid  taken  from  manure.  In  certain  cases  chemotropic 
reactions  result  in  bringing  the  animal  into  regions  of  favorable 
food  conditions,  or  turning  it  away  from  unpleasant  substances. 
Moisture,  which  is  necessary  for  respiration  and  consequently 
for  the  life  of  the  earthworm,  causes  a  positive  reaction,  provided 
it  comes  in  contact  with  the  body,  —  no  positive  reactions  being 
produced  by  chemical  stimulation  from  a  distance.  Negative 
reactions,  on  the  other  hand,  such  as  moving  to  one  side  or  back 
into  the  burrow,  are  produced  even  when  certain  unpleasant 
chemical  agents  are  still  some  distance  from  the  body.  These 
reactions  are  quite  similar  to  those  caused  by  contact  stimuli. 
Danvin  explained  the  preference  of  the  earthworm  for  certain 
kinds  of  food  by  supposing  that  the  discrimination  of  edible  from 
inedible  substance  was  possible  when  in  contact  with  the  body. 
This  would  resemble  the  sense  of  taste  as  present  in  the  higher 
animals.  Such  a  sense  might  account  also  for  the  positive 
burrowing  reaction  cited  above,  which  is  caused  by  contact  with 
fluids  from  manure. 

Certain  experiments  in  which  animals  were  subjected  to  solu- 
tions of  sodium,  ammonium,  lithium,  and  potassium  chlorides 


1 88  AN  INTRODUCTION  TO  ZOOLOGY 

seem  to  show  that  the  worms  are  stimulated  in  different  degrees 
by  them,  since  the  interval  of  time  between  the  application  of  the 
stimulus  and  the  reaction  differs  for  each  substance,  being  shorter 
for  sodium  chloride,  and  successively  longer  for  the  others  in  the 
order  named  (153).  In  man  these  substances  all  taste  practi- 
cally alike,  because  of  the  presence  of  chlorine. 

PHOTOTROPISM.  —  No  definite  visual  organs  have  been  dis- 
covered in  earthworms,  but  nevertheless  these  animals  are  very 
sensitive  to  light,  as  is  proved  by  the  fact  that  a  sudden  illumina- 
tion at  night  will  often  cause  them  to  "  dash  like  a  rabbit  "  into 
their  burrows.  One  investigator  claims  to  have  found  cells  in 
the  ectoderm,  especially  in  the  prostomium  and  posterior  end, 
which  act  as  visual  organs  (144).  The  entire  surface  of  the 
body,  however,  is  sensitive  to  light,  although  the  anterior  region 
is  more  sensitive  than  the  tail,  and  the  middle  less  than  either  of 
the  others.  If  an  animal,  which  is  lying  along  the  ground  with 
its  tail  clinging  to  the  top  of  the  burrow,  is  suddenly  illuminated, 
it  promptly  withdraws  into  its  hole;  if  not  in  touch  with  its  bur- 
row, it  will  crawl  away  from  the  source  of  light.  Very  slight 
differences  in  the  intensity  of  the  light  are  distinguished,  since, 
if  a  choice  of  two  illuminated  regions  is  given,  that  more  faintly 
lighted  is,  in  the  majority  of  cases,  selected.  Thus  far  we  have 
considered  light  as  causing  a  negative  response;  but  a  positive 
reaction  to  faint  light  has  been  demonstrated  for  Allolobophora 
fatida  (135).  This  positive  phototropism  to  faint  light  may 
account  for  the  emergence  of  the  worms  from  their  burrows  at 
night.  Experiments  with  lights  of  different  colors  show  that  red 
is  preferred  to  any  other,  green  being  next,  and  blue  last,  and  that 
the  intensity  of  the  colored  rays  determines  the  effect  (156). 

COMBINATIONS  AND  INTERFERENCE  OF  STIMULI.  —  It  has  been 
shown  that  contact  and  chemical  stimuli  may  combine,  as  in 
the  case  of  the  burrowing  reaction  of  Allolobophora  fcetida.  In 
other  instances  stimuli  interfere  with  one  another;  for  example, 
light  calls  forth  no  reaction  if  the  animals  are  feeding  or 
mating. 


FIG.  96.  Regeneration  in  the  earthworm.  A,  head  end  of  five  segments 
regenerated  from  the  posterior  piece  of  a  worm  ;  B,  tail  regenerated  from 
the  posterior  piece  of  a  worm ;  C,  tail  regenerated  from  an  anterior 
piece  of  a  worm.  (From  Morgan.) 


THE  EARTHWORM  AND  ANNELIDS  IN   GENERAL      189 

PHYSIOLOGICAL  STATE. — From  the  foregoing  account  it  might 
be  inferred  that  only  external  stimuli  are  factors  in  the  behavior  of 
the  earthworm.  This,  however,  is  not  the  case,  since  the  physio- 
logical condition,  which  depends  largely  upon  previous  stimula- 
tion, determines  the  character  of  the  response.  Different  physio- 
logical states  may  be  recognized,  ranging  from  a  state  of  rest  in 
which  slight  stimuli  are  not  effective,  to  a  state  of  great  excite- 
ment caused  by  long-continued  and  intense  stimulation,  in  which 
condition  slight  stimuli  cause  violent  responses  (145). 

Regeneration.  —  A  general  account  of  this  phenomenon  has 
already  been  given  on  pages  138-139,  and  this  should  be  read  in 
order  that  the  following  paragraphs  may  be  perfectly  clear.  If 
the  anterior  portion  of  an  earthworm  is  cut  off  at  any  point  be- 
tween the  end  of  the  prostomium  and  the  fifteenth  to  the  eight- 
eenth segment,  a  new  anterior  end  will  grow  out  from  the  cut 
end  of  the  body.  The  piece  regenerated  will  consist  of  one  seg- 
ment, if  only  one  segment  is  removed;  of  two  segments,  if  two 
segments  are  removed;  of  three,  four,  or  five  segments,  if  three, 
four,  or  five  segments  are  removed;  but  never  more  than  seg- 
ments I  to  V  are  regenerated,  regardless  of  the  number  removed 
(Fig.  96,  A),  and  no  new  reproductive  organs  appear  if  the  origi- 
nal ones  were  contained  in  the  severed  piece.  If  the  cut  is  made 
behind  segment  XVIII,  a  tail  will  grow  out  from  the  cut  surface 
of  the  posterior  piece,  producing  a  worm  consisting  of  two  tails 
joined  at  the  center  (Fig.  96,  B).  Such  a  creature  cannot  take 
in  food,  and  must  slowly  starve  to  death.  When  the  regenerated 
part  is  different  from  the  part  removed,  as  in  the  case  just  cited, 
the  term  heteromorphosis  is  given  to  the  phenomenon. 

If  the  posterior  portion  of  an  earthworm  is  cut  off  at  any  point 
between  the  anal  segment  and  the  twelfth  to  the  fifteenth  seg- 
ment, a  new  tail  will  grow  out  from  the  cut  surface  of  the  part 
remaining  (Fig.  96,  C).  Regeneration  of  a  tail  differs  from  that 
of  a  head,  since  more  than  five  segments  are  replaced.  The  anal 
segment  develops  first,  and  then  a  number  of  new  segments 
are  introduced  between  it  and  the  old  tissue. 


AN  INTRODUCTION  TO  ZOOLOGY 

The  rate  of  regenerative  growth  depends  upon  the  amount  of 
old  tissue  removed.  Thus,  if  only  a  few  segments  of  the  posterior 
end  are  cut  off,  a  new  tail  regenerates  very  slowly;  if  more  is 
removed,  the  new  tissue  is  added  more  rapidly.  In  fact,  the  rate 
of  growth  increases  up  to  a  certain  point  as  the  amount  removed 
increases.  The  factors  regulating  the  rate  of  regeneration  have 
not  yet  been  fully  determined,  although  several  possible  explana- 
tions have  been  suggested. 

Grafting.  —  Pieces  of  earthworms  may  be  grafted  upon 
other  worms  without  much  difficulty.  Several  results  are 
shown  in  Figure  97.  Three  pieces  may  be  so  united  as  to 
produce  a  very  long  worm  (A);  the  tail  of  one  animal  may 
be  grafted  upon  the  side  of  another,  producing  a  double- 
tailed  worm  (B);  or  the  anterior  end  of  one  individual  may 
be  united  with  that  of  another  (C).  In  all  these  experiments 
the  parts  were  held  together  by  threads  until  they  became 
united. 

2.   ANNELIDS  IN  GENERAL 

The  two  chief  classes  of  Annelids  are  the  Chaetopoda  or  Earth- 
worms and  marine  Annelids,  and  the  Hirudinea  or  Leeches. 
Class  Chaetopoda  may  be  divided  into  two  subclasses,  the  Oli- 
gochaeta,  to  which  the  earthworm  belongs,  and  the  Polychaeta. 
Table  VI  contrasts  the  characters  of  these  two  divisions,  and 
Figure  98  shows  one  of  the  most  common  Polychaets,  the  marine 
annelid,  Nereis. 

The  members  of  the  Class  Hirudinea  have  neither  setae  nor 
parapodia.  They  move  by  means  of  a  ventral  sucker  near  the 
posterior  end,  and  a  sucker  mouth  on  the  ventral  surface  at  the 
anterior  end.  They  are  flattened  dorso-ventrally.  The  grooves 
on  the  outside  of  the  body  do  not  correspond  in  number  or  posi- 
tion to  the  septa  within,  there  being  several  external  rings  to  each 
internal  segment.  The  ccelom  is  not  as  large  as  in  the  Chaetopods, 
being  reduced  to  the  cavities  in  which  the  germ  cells  lie,  and  a 


FIG.  97.  Grafting  in  the  earthworm.  A,  union  of  three  pieces  to  make  a 
long  worm ;  B,  union  of  two  pieces  to  make  a  double-tailed  worm ; 
C,  anterior  and  posterior  pieces  united  to  make  a  short  worm.  (From 
Morgan.) 


THE  EARTHWORM  AND  ANNELIDS  IN  GENERAL        191 

number  of  small  spaces.  Leeches  are  usually  aquatic,  hermaph- 
rodite, and  have  a  direct  development.  Figure  99  shows  a 
common  representative  of  this  class. 


FIG.  98.  A  ma- 
rine Polychaet, 
Nereis.  (From 
Shipley  and 
MacBride,  af- 
ter Oersted.) 


FIG.  99.  A  leech,  Hirudo 
medicinalis.  i,  mouth; 
2,  posterior  sucker;  3, 
sensory  papillae.  (From 
Shipley  and  MacBride.) 


IQ2 


AN  INTRODUCTION  TO  ZOOLOGY 


TABLE  VI 

THE  CHARACTERS  OF  OLIGOOLETS  AND  POLYOLETS  CONTRASTED 


OLIGOCH^ETA 


POLYCH^TA 


No    special    locomotor    protrusions 
(parapodia) ;  few  setae. 

No  other  external  appendages. 


Hermaphrodite. 

Development  direct. 

Aquatic  (fresh-water)  or  terrestrial. 


Parapodia ;  many  setae. 

External  appendages  in  the  form  of 
antennae,  gills,  and  cirri  (see  Fig. 
98). 

Sexes  usually  separate. 

A  metamorphosis  in  development. 

Aquatic  (marine). 


CHAPTER  XI 
THE  CRAYFISH  AND  ARTHROPODS   IN   GENERAL 

i.  THE  CRAYFISH 
(Cambarus  wrilis  Girard)1 

CRAYFISHES  inhabit  fresh-water  lakes,  ponds,  and  streams. 
The  species  Cambarus  mrilis  is  common  in  some  of  the  central 
states  and  Cambarus  affinis  in  the  eastern  part  of  the  country. 
The  lobster  is  so  nearly  like  the  crayfish  in  structure  that  the 
anatomical  portion  of  this  chapter  may  be  applied  also  in  large 
part  to  this  animal.  In  Europe  the  most  common  crayfish  is 
As  focus  fluviatilis,  a  species  made  famous  by  Huxley's  classical 
work  "  The  Crayfish." 

The  crayfish,  Cambarus  mrilis,  is  usually  found  concealed  under 
rocks  or  logs  at  the  bottom  of  ponds  and  streams.  Here  it  lies 
with  its  head  toward  the  entrance  to  its  hiding  place.  When 
crawling  about  or  swimming  in  the  open  water,  its  hard  shell 
helps  protect  it  from  fish,  while  its  color,  which  resembles  the 
bottom,  tends  to  make  its  detection  difficult.  Crayfishes  may  be 
captured  easily  by  hand,  with  a  net,  or  by  fishing  for  them  with  a 
string  baited  with  a  piece  of  meat.  They  thrive  in  an  aquarium, 
and  their  entire  life  history  may  be  observed  in  the  laboratory. 
The  yearly  decrease  in  the  number  of  lobsters  available  for  food, 

1  The  complete  life  history,  and  the  details  of  the  anatomy  are  not  known  for 
any  single  species  of  the  genus  Cambarus.  The  following  account,  therefore, 
must  necessarily  be  a  composite  containing  not  only  observations  on  various 
species  of  Cambarus,  but  also  on  the  European  crayfish,  Astacus  (Potamobius) 
fluviatilis.  The  differences  between  these  crayfishes  are,  however,  so  slight 
as  to  be  of  little  importance  except  in  a  detailed  monograph, 
o  193 


IQ4  AN  INTRODUCTION  TO  ZOOLOGY 

and  the  steadily  increasing  demand  for  crayfishes,  will  undoubt- 
edly soon  make  it  worth  while  to  raise  the  latter  for  market  (162). 

External  Features.  —  EXOSKELETON.  — The  outside  of  the  body 
of  the  crayfish  is  covered  by  an  extremely  hard  chitinous  cuticle 
impregnated  with  lime  salts.  This  exoskeleton  is  thinner  and 
flexible  at  the  joints,  allowing  movement.  A  delicate  cuticle 
of  the  same  substance  (chitin)  was  noted  in  the  earthworm 
(p.  166). 

REGIONS  OF  THE  BODY.  —  Unlike  the  earthworm,  the  body  of 
the  crayfish  shows  two  distinct  regions,  an  anterior  rigid  portion, 
the  cephdothorax,  and  a  posterior  series  of  segments,  the  abdomen. 


FIG.  100.  Transverse  section  through  the  abdomen  of  the  crayfish.  DA, 
dorsal  abdominal  artery ;  EM ,  extensor  muscles  of  the  abdomen ; 
EP,  epimeron  ;  FM ,  flexor  muscles  of  abdomen  ;  M,  muscles  of  append- 
age ;  N,  endopodite ;  NG,  nerve  ganglion ;  P,  protopodite ;  PL. 
pleuron ;  PR,  intestine  ;  S,  sternum  ;  T,  tergum ;  V,  ventral  abdominal 
artery ;  X,  exopodite.  (From  Marshall  and  Hurst.) 

The  entire  body  is  segmented,  but  the  joints  have  been  obliterated 
on  the  dorsal  surface  of  the  cephalothorax. 


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THE  CRAYFISH  AND  ARTHROPODS   IN   GENERAL         195 

STRUCTURE  OF  A  SEGMENT.  —  A  typical  segment  is  shown  in 
cross  section  in  Figure  100.  It  consists  of  the  following  parts:  (i) 
a  convex  dorsal  plate,  the  tergum  (T),  (2)  a  ventral  transverse  bar, 
the  sternum  (S),  (3)  plates  projecting  down  at  the  sides,  the 
pleura  (PL),  and  (4)  smaller  plates  between  the  pleura  and  the 
basis  of  the  limb,  the  epimera  (EP). 

CEPHALOTHORAX.  —  The  cephalothorax  consists  of  segments 
I-XIII,  which  are  inclosed  dorsally  and  laterally  by  a  cuticular 
shield,  the  carapace.  An  indentation,  termed  the  cervical  groove, 
runs  across  the  mid-dorsal  region  of  the  carapace,  and  obliquely 
forward  on  either  side,  separating  the  cephalic  or  head  region 
from  the  posterior  thoracic  portion.  The  Anterior  pointed  exten- 
sion of  the  carapace  is  known  as  the  rostrum.  Beneath  this  on 
either  side  is  an  eye  at  the  end  of  a  movable  peduncle.  The  mouth 
is  situated  on  the  ventral  surface  near  the  posterior  end  of  the 
head  region.  It  is  partly  obscured  by  the  neighboring  append- 
ages. The  carapace  of  the  thorax  is  separated  by  branchio- 
cardiac  grooves  into  three  parts,  a  median  dorsal  longitudinal 
strip,  the  areola,  and  two  large  convex  flaps,  one  on  either  side, 
the  branchiostegites,  which  protect  the  gills  beneath  them. 

ABDOMEN.  —  In  the  abdomen  there  are  six  segments,  and  a  ter- 
minal extension,  the  telson,  bearing  on  its  ventral  surface  the  longi- 
tudinal anal  opening.  Whether  or  not  the  telson  is  a  true  seg- 
ment is  still  in  dispute;  we  shall  adopt  the  view  that  it  is  not. 
The  first  abdominal  segment  (XIV)  is  smaller  than  the  others  and 
lacks  the  pleurae.  Segments  XV-XIX  are  like  the  type  described 
above. 

APPENDAGES.  —  With  the  possible  exception  of  the  first  ab- 
dominal segment  in  the  female,  every  segment  of  the  body  bears 
a  pair  of  jointed  appendages.  These  are  all  variations  of  a  com- 
mon type  (Fig.  100),  consisting  of  a  basal  segment,  the  protopo- 
dite  (P),  which  bears  two  branches,  an  inner  endopodite  (N), 
and  an  outer  exopodite  (X).  Beginning  at  the  anterior  end,  the 
appendages  are  arranged  as  follows  (Fig.  101).  In  front  of  the 
mouth  are  (I)  the  antennules,  and  (II)  the  antenna;  the  mouth 


AN  INTRODUCTION  TO  ZOOLOGY 


--ex. 


FIG.  102.  Types  of  crayfish  appendages. 
A,  second  maxilla,  foliaceous  type ; 
1-4,  basopodite ;  5,  endopodite  ; 
6,  exopodite  ;  ep.,  scaphognathite. 
(From  Cambridge  Natural  His- 
tory.) B,  swimmeret,  biramous 
type;  ex.  bs.,  protopodite; 
ex.,  exopodite ;  en.,  endopodite. 
(From  Lankester's  Treatise.) 
C,  second  walking  leg,  uniramous 
type ;  cxp.  bp.,  protopodite ;  ip., 
mp.,  cp.,  pp.,  dp.,  segments  of  en- 
dopodite ;  ep.,  epipodite.  (From 
Cambridge  Natural  History.) 


possesses  (III)  a  pair  of  man- 
dibles, behind  which  are  (IV) 
the  first,  and  (V)  the  second 
maxillae;  the  thoracic  region 
bears  (VI)  the  first,  (VII)  the 
second,  and  (VIII)  the  third 
maxillipedes,  (IX)  the  pin- 
chers or  chelipeds,  and  (X- 
XIII)  four  other  pairs  of 
walking  legs;  beneath  the 
abdomen  are  six  (XIV-XIX) 
pairs  of  sivimmerets,  some  of 
which  are  much  modified. 
Table  VII  gives  brief  de- 
scriptions of  the  different  ap- 
pendages, and  shows  the 
modifications  due  to  dif- 
ferences in  function.  The 
functions  of  some  of  the  ap- 
pendages are  still  in  doubt. 

Three  kinds  of  appendages 
can  be  distinguished  in  the 
adult  crayfish;  (i)  the  folia- 
ceous,  e.g.  the  second  maxilla 
(Fig.  102,  A),  (2)  the  bira- 
mous,e.g.  the  swimmerets  (Fig. 
1 02,  B),  and  (3)  the  unira- 
mous, e.g.  the  walking  legs 
(Fig.  102,  C).  All  of  these 
appendages  have  doubtless 
been  derived  from  a  single 
type,  the  modifications  being 
due  to  the  functions  per- 
formed by  them.  The 
biramous  type  probably 


THE   CRAYFISH  AND   ARTHROPODS  IN  GENERAL          197 


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200  AN  INTRODUCTION  TO  ZOOLOGY 

represents  the  condition  from  which  the  other  types  developed. 
The  uniramous  walking  legs,  for  example,  pass  through  a 
biramous  stage  during  their  embryological  development.  Again, 
the  biramous  embryonic  maxillipedes  are  converted  into  the 
foliaceous  type  by  the  expansion  of  their  basal  segments  (166). 

General  Internal  Anatomy.  —  The  body  of  the  crayfish 
contains  all  of  the  important  systems  of  organs  characteristic  of 
the  higher  animals.  The  ccelom  is  not  a  large  cavity,  as  in  the  earth- 
worm, but  is  restricted  to  the  cavities  of  the  reproductive  organs. 
Certain  of  the  organs  are  metamerically  arranged,  e.g.  the  nerv- 
ous system;  others  like  the  excretory  organs,  are  concentrated 
into  a  small  space.  The  systems  of  organs  and  their  functions 
will  be  presented  in  the  following  order:  (i)  digestive,  (2)  vas- 
cular, (3)  respiratory,  (4)  excretory,  (5)  nervous,  (6)  sense  organs, 
(7)  muscular,  and  (8)  reproductive. 

Digestive  System.  —  The  alimentary  canal  of  Cambarus  con- 
sists of  the  following  parts  (Fig.  101) :  — 

(1)  The  mouth  (4)  opens  on  the  ventral  surface  between  the 
jaws  (3). 

(2)  The  oesophagus  (20)  is  a  short  tube  leading  from  the  mouth 
to  the  stomach. 

(3)  The  stomach  is  a  large  cavity  divided  by  a  constriction 
into  an  anterior  cardiac  chamber  (21)  and  a  smaller  posterior 
pyloric  chamber  (22).     In  the  stomach  are  a  number  of  chitinous 
ossicles  of  use  in  chewing  the  food,  and  collectively  known  as  the 
gastric  mill.     The  most  important  of  these  are  (a)  a  median  car- 
diac ossicle,  (b)  a  median  urocardiac  ossicle,  (c)  two  lateral  ptero- 
cardiac  ossicles,  (d)  a  pair  of  lateral  zygocardiac  ossicles,  (e)  a 
pyloric  ossicle,  and  (/)  a  prepyloric  ossicle.     The  ossicles  are  able 
to  move  one  upon  another,  and,  being  connected  with  powerful 
muscles,  are  effective  in  grinding  up  the  food.     On  either  side  of 
the  pyloric  chamber  enters  a  duct  of  the  digestive  glands,  and 
above  is  the  opening  of  the  small  caecum. 

At  certain  times  two  calcareous  bodies,  known  as  gastroliths, 
are  present  in  the  lateral  walls  of  the  cardiac  chamber  of  the 


THE   CRAYFISH  AND  ARTHROPODS   IN   GENERAL        2OI 

stomach.  Their  function  is  not  certain,  but  is  probably  for  the 
storage  of  the  calcareous  matter  used  in  hardening  the  exoskeleton. 

(4)  The  intestine  (24)  is  a  small  tube  passing  posteriorly  near 
the  dorsal  wall  of  the  abdomen,  and  opening  to  the  outside 
through  the  anus  (6)  on  the  ventral  surface  of  the  telson  (7). 

The  digestive  glands,  or  "  liver,"  are  situated  in  the  thorax, 
one  on  either  side.  Each  consists  of  three  lobes  composed  of  a 
great  number  of  small  tubules.  The  glandular  epithelium  lining 
these  tubules  produces  a  secretion  which  passes  into  the  hepatic 
ducts  and  thence  into  the  pyloric  chamber  of  the  stomach. 

Nutrition.  —  FOOD.  —  The  food  of  the  crayfish  is  made  up 
principally  of  living  animals  such  as  snails,  tadpoles,  insect  larvae, 
small  fish,  and  frogs;  but  decaying  organic  matter  is  also  eaten. 
Not  infrequently  crayfishes  prey  upon  others  of  their  kind.  They 
feed  at  night,  being  more  active  at  dusk  and  daybreak  than  at 
any  other  time  (167).  Their  method  of  feeding  may  be  observed 
in  the  laboratory  if  a  little  fresh  meat  is  offered  to  them.  The 
maxillipedes  and  maxillae  hold  the  food  while  it  is  being  crushed 
into  small  pieces  by  the  mandibles.  It  then  passes  through  the 
oesophagus  into  the  stomach.  The  coarser  parts  are  ejected 
through  the  mouth. 

DIGESTION.  —  In  the  cardiac  chamber  of  the  stomach,  the  food 
is  ground  up  by  the  ossicles  of  the  gastric  mill.  When  fine  enough, 
it  passes  through  the  strainer  which  lies  between  the  two  divisions 
of  the  stomach.  This  strainer  consists  of  two  lateral  and  a 
median  ventral  fold  which  bear  hairlike  setae,  and  allow  the  pas- 
sage of  only  liquids  or  very  fine  particles.  In  the  pyloric  cham- 
ber, the  food  is  mixed  with  the  secretion  from  the  digestive  glands 
brought  in  by  way  of  the  hepatic  ducts.  From  the  pyloric 
chamber  the  dissolved  food  passes  into  the  intestine  by  the  walls 
of  which  the  nutritive  fluids  are  absorbed.  Undigested  particles 
pass  on  into  the  posterior  end  of  the  intestine,  where  they  are 
gathered  together  into  faeces,  and  egested  through  the  anus. 

Vascular  System.  —  THE  BLOOD.  —  The  blood  into  which  the 
absorbed  food  passes  is  an  almost  colorless  liquid  in  which  are 


202  AN  INTRODUCTION  TO  ZOOLOGY 

suspended  a  number  of  ameboid  cells,  the  blood  corpuscles  or 
amebocytes.  The  principal  functions  of  the  blood  are  the 
transportation  of  food  materials  from  one  part  of  the  body  to 
another,  of  oxygen  from  the  gills  to  the  various  tissues,  of  carbon 
dioxide  to  the  gills,  and  of  urea  to  the  excretory  organs.  If  a 
crayfish  is  wounded,  the  blood,  on  coming  in  contact  with  the  air, 
thickens,  forming  a  clot.  It  is  said  to  coagulate.  This  clogs  the 
opening  and  prevents  loss  of  blood.  The  chelipeds  and  other 
walking  legs  of  the  crayfish  have  a  breaking  point  near  their 
bases.  When  one  is  injured  the  animal  may  break  the  limb  at 
this  point  and  lessen  the  blood  flow,  since  only  a  small  space  is 
present  in  the  appendage  at  this  particular  spot,  and  coagulation, 
therefore,  takes  place  very  quickly. 

BLOOD  VESSELS.  —  The  principal  blood  vessels  are  a  heart, 
seven  main  arteries,  and  a  number  of  spaces  called  sinuses. 

HEART.  —  The  heart  (Fig.  101,  2p)  is  a  muscular-walled,  saddle- 
shaped  sac  lying  in  the  pericardial  sinus  in  the  median  dorsal  part 
of  the  thorax.  It  may  be  considered  as  a  dilatation  of  a  dorsal 
vessel  resembling  that  of  the  earthworm.  Six  elastic  ligaments, 
two  anterior,  two  posterior,  and  two  running  along  the  ventral 
border  of  each  lateral  surface,  fasten  it  to  the  walls  of  the  peri- 
cardial sinus.  Three  pairs  of  valvular  apertures,  called  ostia, 
one  dorsal  and  two  lateral,  allow  the  blood  to  enter  from  the 
surrounding  sinus  (174). 

ARTERIES.  —  Five  arteries  arise  from  the  anterior  end  of  the 
heart  (Fig.  101). 

(i)  The  ophthalmic  artery  (34)  is  a  median  dorsal  tube  passing 
forward  over  the  stomach,  and  supplying  the  cardiac  portion, 
the  oesophagus,  and  head. 

(2,  3)  The  two  antennary  arteries  (35)  arise  one  on  each  side 
of  the  ophthalmic  artery,  pass  forwards,  outwards,  and  down- 
wards, and  branch,  sending  a  gastric  artery  to  the  cardiac  part 
of  the  stomach,  arteries  to  the  antennse,  to  the  excretory  organs, 
and  to  the  muscles  and  other  cephalic  tissues. 

(4,  5)    The  two   hepatic  arteries  (36)  leave  the  heart  below 


THE   CRAYFISH  AND   ARTHROPODS  IN    GENERAL        203 

the  antennary  arteries.  They  lead  directly  to  the  digestive 
glands. 

A  single  dorsal  abdominal  artery  arises  from  the  posterior 
end  of  the  heart. 

(6)  The  dorsal  abdominal  artery  (31)  is  a  median  tube  leading 
backwards  from  the  ventral  part  of  the  heart,  and  supplying  the 
dorsal  region  of  the  abdomen.  It  branches  near  its  point  of 
origin,  giving  rise  to  the  sternal  artery  (jo) ;  this  leads  directly 
downward,  and,  passing  between  the  nerve  cords  connecting  ths 
fourth  and  fifth  pairs  of  thoracic  ganglia  (see  p.  205)  divides  into 
two  arteries.  One  of  these,  the  -ventral  thoracic  artery,  runs  for- 
ward beneath  the  nerve  chain,  and  sends  branches  to  the  ventral 
thoracic  region,  and  to  appendages  III  to  XIII;  the  other, 
the  ventral  abdominal  artery  (32),  runs  backward  beneath  the 
nerve  chain,  and  sends  branches  to  the  ventral  abdominal  region 
and  to  the  abdominal  appendages. 

SINUSES.  —  The  blood  passes  from  the  arteries  into  spaces  lying 
in  the  midst  of  the  tissues,  called  sinuses.  The  pericardial  sinus 
has  already  been  mentioned.  The  thorax  contains  a  large  ventral 
blood  space,  the  sternal  sinus,  and  a  number  of  branchio-cardiac 
canals  extending  from  the  bases  of  the  gills  to  the  pericardial 
sinus.  A  perimsceral  sinus  surrounds  the  alimentary  canal  in 
the  cephalothorax. 

BLOOD  FLOW. — The  heart,  by  means  of  rhythmical  contrac- 
tions, forces  the  blood  through  the  arteries  to  all  parts  of  the  body. 
Valves  are  present  in  every  artery  where  it  leaves  the  heart; 
they  prevent  the  blood  from  flowing  back.  The  finest  branches 
of  these  arteries,  the  capillaries,  open  into  spaces  between  the 
tissues,  and  the  blood  eventually  reaches  the  sternal  sinus. 
From  here  it  passes  into  the  efferent  channels  of  the  gills  and  into 
the  gill  filaments,  where  the  carbonic  acid  in  solution  is  exchanged 
for  oxygen  from  the  water  in  the  branchial  chambers.  It  then 
returns  by  way  of  the  afferent  gill  channels,  passes  into  the  bran- 
chio-cardiac sinuses,  thence  to  the  ptricardial  sinus,  and  finally 
through  the  ostia  into  the  heart.  The  valves  of  the  ostia  allow 


204  AN  INTRODUCTION  TO  ZOOLOGY 

the  blood  to  enter  the  heart,  but  prevent  it  from  flowing  back  into 
the  pericardial  sinus. 

Respiratory  System.  —  Between  the  branchiostegites  and  the 
body  wall  are  the  branchial  chambers  containing  the  gills.  At 
the  anterior  end  of  the  branchial  chamber  is  a  channel  in  which  the 
scaphognathite  of  the  second  maxilla  (Fig.  102,  A,  ep.)  moves  back 
and  forth,  forcing  the  water  out  through  the  anterior  opening. 
Water  flows  in  through  the  posterior  opening  of  the  branchial 
chamber. 

GILLS.  —  There  are  two  rows  of  gills,  named  according  to  their 
points  of  attachment.  The  outermost,  the  podobranchia,  are 
fastened  to  the  coxopodites  of  certain  appendages  (see  Table 
VIII)  and  the  inner  double  row,  the  arthrobranchia,  arise  from 
the  membranes  at  the  bases  of  these  appendages.  In  Astacus 
there  is  a  third  row,  the  pleurobranckice,  attached  to  the  walls  of 
the  thorax.  The  number  and  arrangement  of  these  gills  are 
shown  in  Table  VIII  (169).  The  podobranchiae  consist  of  a 

TABLE  VIII 

THE    NUMBER    AND    POSITION    OF    THE    GILLS    OF    THE    CRAYFISH 

(Cambarns) 

SEGMENT        PODOBRAN-  ARTHROBRANCHI.E  TOTAL 

CHLE  NUMBER 

Anterior  Posterior 

VI  o  (ep.)  ooo  (ep.) 


VII  i 

VIII  i 

LX  i 

X  i 

XI  i 

XII  i 


o  2 

3 
3 
3 
3 
3 


6(ep). 


basal  plate  covered  with  delicate  setae  and  a  central  axis  bearing 
a  thin,  longitudinally  folded  corrugated  plate  on  its  distal  end, 
and  a  feather-like  group  of  branchial  filaments.  The  arthrobran- 
chiae  have  a  central  stem  on  either  side  of  which  extends  a  number 
of  filaments,  causing  the  entire  structure  to  resemble  a  plume. 


THE  CRAYFISH  AND  ARTHROPODS  IN   GENERAL        205 

Attached  to  the  base  of  the  first  maxilliped  is  a  broad  thin  plate, 
the  epipodite,  which  has  lost  its  branchial  filaments. 

Excretory  System.  —  The  excretory  organs  are  a  pair  of  rather 
large  bodies,  the  "green  glands"  (Fig.  101,  40)  situated  in  the 
ventral  part  of  the  head  anterior  to  the  oesophagus.  Each  green 
gland  consists  of  a  glandular  portion,  green  in  color  (40),  a  thin- 
walled  dilatation,  the  bladder  (41),  and  a  duct  opening  to  the 
exterior  through  a  pore  at  the  top  of  the  papilla  on  the  basal 
segment  of  the  antenna  (42). 

Nervous  System.  —  The  morphology  of  the  nervous  system  of 
the  crayfish  is  in  many  respects  similar  to  that  of  the  earthworm. 
The  central  nervous  system  includes  a  dorsal  ganglionic  mass,  the 
brain  (Fig.  101,  25),  in  the  head,  and  two  circum-cesophageal 
connectives  (26]  passing  to  the  ventral  nerve  cord  (27),  which  lies 
near  the  median  ventral  surface  of  the  body. 

BRAIN.  — The  brain  (Fig.  101,  25)  is  a  compact  mass  larger 
than  that  of  the  earthworm,  and  supplies  the  eyes,  antennules,  and 
antennae  with  nerves. 

VENTRAL  NERVE  CORD.  —  The  ganglia  and  connectives  of  the 
ventral  nerve  cord  are  more  intimately  fused  than  in  the  earth- 
worm, and  it  is  difficult  to  make  out  the  double  nature  of  the 
connectives,  except  between  segments  XI  and  XII,  where  the 
sternal  artery  passes  through  (Fig.  101,  jo).  Each  segment 
posterior  to  VII  possesses  a  ganglionic  mass,  which  sends  nerves 
to  the  surrounding  tissues.  The  large  sub-cesophageal  ganglion  in 
segment  VII  consists  of  the  ganglia  of  segments  III- VII  fused 
together.  It  sends  nerves  to  the  mandibles,  maxillae,  and  first 
and  second  maxillipeds. 

The  VISCERAL  NERVOUS  SYSTEM  consists  of  an  anterior  visceral 
nerve  which  arises  from  the  ventral  surface  of  the  brain,  is  joined 
by  a  nerve  from  each  circum-cesophageal  connective,  and,  passing 
back,  branches  upon  the  dorsal  wall  of  the  pyloric  part  of  the 
stomach,  sending  a  lateral  nerve  on  each  side  to  unite  with  an 
infer o-lateral  nerve  from  the  stomato gastric  ganglion. 

Sense  Organs.  —  EYES,  —  The  eyes  of  the  crayfish  are  situated 


2O6 


AN  INTRODUCTION  TO  ZOOLOGY 


at  the  end  of  movable  stalks  which  extend  out,  one  from  each  side 
of  the  rostrum  (Fig.  101,  28).     The  external  convex  surface  of 

the  eye  is  covered  by  a  modified 
portion  of  the  transparent  cuticle, 
called  the  cornea.  This  cornea  is 
divided  by  a  large  number  of  fine 
lines  into  four-sided  areas,  termed 
facets.  Each  facet  is  but  the  ex- 
ternal part  of  a  long  visual  rod 
known  as  an  ommatidium  (Fig.  103). 
Sections  show  the  compound  eye 
to  be  made  up  of  similar  omma- 
tidia  lying  side  by  side,  but  sepa- 
rated from  one  another  by  a  layer 
of  dark  pigment  cells.  The  average 
number  of  ommatidia  in  a  single  eye 
is  2500  (180). 

Two  ommatidia  are  shown  in  Fig- 
ure 103.  Beginning  at  the  outer 
surface,  each  ommatidium  consists  of 
the  following  parts :  (a)  a  corneal 
facet  (i),  (b)  two  corneagen  cells 
(2}  which  secrete  the  cornea,  (c)  a 
crystalline  cone  (5)  formed  by  four 
cone  cells,  or  vitrella  (j),  (d)  two 
retinular  cells  surrounding  the  cone 
(not  shown  in  Fig.  103),  (e)  seven 

FIG.  103.  Longitudinal  sections  of  two  om- 
matidia of  the  crayfish ;  A,  pigment 
arranged  as  influenced  by  light ;  B, 
pigment  arranged  as  influenced  by 
darkness ;  i,  cornea ;  2,  nucleus  of 
corneagen  cells  ;  j,  nucleus  of  vitrella  ; 

4,  nucleus  of  pigment   cell ;     5,  crystalline   cone ;     6,  tapetum   cell ; 

9,  basement  membrane;     10,  retinal 


B 


7,  rhabdom ;     8,  retinal   cell ; 

nerve  fiber.     (From  Sedgwick  after  Parker.) 


THE  CRAYFISH  AND  ARTHROPODS  IN  GENERAL 


207 


retinular  cells  (8)  which  form  a  rhabdom  (7)  consisting  of  four 
rhabdomeres,  and  (/)  a  number  of  pigment  cells  (4,  6).  Fibers 
from  the  optic  nerve  enter  at  the  base  of  the  ommatidium  and 
communicate  with  the  inner  ends  of  the  retinular  cells  (10). 

VISION.  —  The  eyes  of  the  crayfish  are  supposed  to  produce 
an  erect  mosaic  or  "apposition  image";  this  is  illustrated  in  Fig- 
ure 104  where  the  ommatidia  are  represented  by  a-e  and  the  fibers 
from  the  optic  nerve  by  a'-e'.  The  rays  of  light  from  any  point 
a,  b,  or  c,  will  all  encounter  the  dark  pigment  cells  surrounding  the 
ommatidia  and  be  absorbed,  except  the  ray  which  passes  directly 
through  the  center  of  the  cornea  as  d  or  e ;  this  ray  will  penetrate 
to  the  retinulae,  and  thence  to  the  fibers  from  the  optic  nerve. 
"  Thus  the  retinula  of  one  ommatidium  receives  a  single  resultant 
impression  from  the  light  which  reaches  it.  But  the  adjacent 
ommatidia  being  directed  to  a  different,  though  adjoining,  region 
of  the  outer  world,  may  transmit  a  different  impression,  and  the 
stimuli  from  all  the  omma- 
tidia  which  make  up  a  com- 
pound eye  will  correspond  in 
greater  or  less  degree  to  the 
whole  of  the  visible  outer 
world  which  subtends  their 
several  optic  axes.  The  sum 
of  the  resulting  images  which 
we  may  thus  suppose  to  be  FIG.  104.  Diagram  to  explain  mosaic 
transmitted  to  the  brain  may  vision  (see  text)  (From  Packard 
be  compared  to  a  mosaic  in 

which  the  effect  is  given  by  a  large  number  of  separate  pieces, 
of  one  size  and  each  of  uniform  colour  "  (183,  p.  332).  This 
method  of  image  formation  is  especially  well  adapted  for  record- 
ing motion,  since  any  change  in  the  position  of  a  large  object 
affects  the  entire  2500  ommatidia. 

When  the  pigment  surrounds  the  ommatidia  (Fig.  103,  A), 
vision  is  as  described  above;  but  it  has  been  found  that  in  dim 
light  the  pigment  migrates  partly  toward  the  outer  and  partly 


208  AN  INTRODUCTION  TO  ZOOLOGY 

toward  the  basal  end  of  the  ommatidia  (Fig.  103,  B).  When  this 
occurs  the  ommatidia  no  longer  act  separately,  but  a  combined 
image  is  thrown  on  the  retinular  layer.  "  In  this  manner  an 
erect '  superposition  image '  is  formed,  the  rays  refracted  by  a  large 
number  of  crystalline  cones  being  superposed  at  the  focus  on  the 
retina,  and  a  stimulus  far  stronger  in  proportion  to  the  intensity 
of  the  illumination  than  that  of  the  apposition  image,  though 
probably  much  less  distinct  in  detail,  is  given  to  the  retinulae  " 

(183,  p.  333). 

STATOCYSTS.  — The  statocysts  of  Cam  jams  are  chitinous- walled 
sacs  situated  one  in  the  basal  segment  of  each  antennule.  In  the 
base  of  the  statocyst  is  a  ridge,  called  the  sensory  cushion,  and 
three  sets  of  hairs,  over  two  hundred  in  all,  each  innervated  by  a 
single  nerve  fiber.  Among  these  hairs  are  a  number  of  large 
grains  of  sand,  the  statoliths,  which  are  placed  there  by  the  cray- 
fish. Beneath  the  sensory  cushion  are  glands  which  secrete  a  sub- 
stance for  the  attachment  of  the  statoliths  to  the  hairs  (181). 

The  statocyst  for  many  years  was  considered  an  auditory 
organ,  and  it  may  possibly  function  as  such,  though  recent 
investigations  have  proven  that  it  is  primarily  an  organ  of 
equilibration.  The  contact  of  the  statoliths  with  the  statocyst 
hairs  determines  the  orientation  of  the  body  while  swimming, 
since  any  change  in  the  position  of  the  animal  causes  a  change 
in  the  position  of  the  statoliths  under  the  influence  of  gravity. 
When  the  crayfish  changes  its  exoskeleton  in  the  process  of  molt- 
ing, the  statocyst  is  also  shed.  Individuals  that  have  just  molted, 
or  have  had  their  statocysts  removed,  lose  much  of  their  powers 
of  orientation.  Perhaps  the  most  convincing  proof  of  the  func- 
tion of  equilibration  is  that  furnished  by  the  experiments  of 
Kreidl  (172).  This  investigator  placed  shrimps,  which  had  just 
molted  and  were  therefore  without  statoliths,  in  filtered  water. 
When  supplied  with  iron  filings,  the  animals  filled  their  statocysts 
with  them.'  A  strong  electro-magnet  was  then  held  near  the 
statocyst,  and  the  shrimp  took  up  a  position  corresponding  to  the 
resultant  of  the  two  pulls,  that  of  gravity  and  of  the  magnet. 


THE  CRAYFISH  AND  ARTHROPODS  IN  GENERAL 


209 


ex*- 


Muscular  System.  —  The  principal  muscles  in  the  body  of  the 
crayfish  are  situated  in  the  abdomen,  and  are  used  to  bend  that 
part  of  the  animal  forward  upon 
the  ventral  surface  of  the  thorax, 
thus  producing  backward  loco- 
motion in  swimming.  Other 
muscles  extend  the  abdomen  in 
preparation  for  another  stroke. 
Figure  100  shows  a  cross  section 
of  an  abdominal  segment.  The 
powerful  flexor  muscles  (FM) 
occupy  almost  the  entire  abdomi- 
nal space.  In  the  dorsal  region 
are  the  less  powerful  extensor 
muscles  (EM).  Other  muscles  of 
considerable  size  are  situated 
within  the  tubular  appendages, 
especially  the  chelipeds.  Figure 
105  shows  how  the  muscles  of  a 
walking  leg  are  arranged.  A 
comparison  of  the  skeleton  and 
muscles  of  the  crayfish  with  those 
of  man  is  interesting.  The  skele- 
ton of  the  crayfish  is  external 
and  tubular,  except  in  the  ven- 
tral part  of  the  thorax  (Fig.  101, 
39).  The  muscles  are  internal, 
and  attached  to  the  inner  surface 
of  the  skeleton.  In  man,  on  the 
other  hand,  the  skeleton  is  in- 
ternal and  the  muscles  external. 

Reproductive  System.  —  Cray- 
fishes    are      normally     dioecious, 
there  being  only  a  few  cases  on 
record    where     both    male    and 
p 


FIG.  105.  Part  of  the  leg  of  a 
crayfish  showing  muscles. 
(From  Parker  and  Haswell.) 


210 


AN  INTRODUCTION  TO  ZOOLOGY 


female  reproductive  organs 
were  found  in  a  single  speci- 
men. 

MALE  REPRODUCTIVE  OR- 
GANS.—  The  male  organs  (Fig. 
106)  consist  of  a  testis  and  two 
vasa  deferentia  (j)  which  open 
through  the  coxopodites  of  the 
fifth  pair  of  walking  legs  (4, 
5).  The  testis  lies  just  be- 
neath the  pericardial  sinus.  It 
is  a  soft  white  body  possessing 
two  anterior  lobes  (Fig.  106,  /) 
and  a  median  posterior  exten- 
sion (2).  The  vasa  deferentia 
are  long  coiled  tubes. 


FIG.  106.  Male  reproductive  organs  of 
the  crayfish.  i,  right  anterior 
lobe  of  testis  ;  2,  posterior  lobe  of 
testis  ;  j,  vas  deferens  ;  4,  external 
opening  of  vas  deferens  ;  6,  base 
of  fifth  walking  leg.  (From  Ship- 
ley and  MacBride.) 


SPERMATOGENESIS.  - 
The  primitive  germ  cells 
within  the  testis  pass 
through  two  maturation 
divisions,  and  then  met- 
amorphose into  sperma- 
tozoa. These  are  flat- 
tened spheroidal  bodies 

when  inclosed  within  the 
.107.    A  spermatozoon  of  the  crayfish.    A  ,   -  , 

testis  or  vas  deferens,  but 
(From  Andrews,  in  Anat.  Anz.) 

if  examined  in  water  or 

some  other  liquid  they  are  seen  to  uncoil,  finally  becoming  star- 
shaped  (Fig.  107). 


FIG 


THE  CRAYFISH  AND  ARTHROPODS  IN  GENERAL 


The  spermatozoa  remain  in  the  testis  and  vasa  deferentia 
until  copulation  takes  place.  As  many  as  two  million  sperma- 
tozoa are  contained  in  the  vasa  deferentia  of  a  single  specimen 

(158). 

FEMALE  REPRODUCTIVE  ORGANS.  —  The  ovary  resembles 
the  testis  in  form,  and  is  similarly  located  in  the  body 
(Fig.  1 08).  A  short  oviduct  (i)  leads  from  near  the 
center  of  each  side  of  the  ovary  to  the  external  aperture  in  the 
coxopodite  of  the  third  walking  leg  (4,  5). 

OOGENESIS.  — The  primitive  germ  cells  in  the  walls  of  the  ovary 
grow  in  size,  become  sur- 
rounded by  a  layer  of 
small  cells,  the  follicle, 
which  eventually  break 
down,  allowing  the  eggs  to 
escape  into  the  central 
cavity  of  the  ovary.  At 
the  time  of  laying  the  ova 
pass  out  through  the  ovi- 
duct. 

Breeding  Habits.  —  The 
details  of  copulation,  egg- 
laying,  and  the  larval 
stages  of  Cambarus  have 
only  recently  been  made 
out,  and  even  now  our 
account  must  be  derived  FlG-  Io8-  Female  reproductive  organs 

of   the   crayfish,      i,  right    oviduct ; 


from  observations  of 
several  different  species, 
since  the  entire  life  his- 
tory of  a  single  species 
has  never  been  recorded. 
The  development  of  the  eggs  of  Cambarus  from  the  time  of 
deposition  to  the  time  of  hatching  has  likewise  never  been 
investigated. 


2,  right  lobe  of  ovary ;  j,  left  lobe 
opened  to  show  central  cavity  ;  4,  ex- 
ternal opening  of  oviduct ;  5,  base  of 
third  walking  leg.  (From  Shipley  and 
MacBride.) 


212  AN  INTRODUCTION  TO  ZOOLOGY 

The  principal  events  in  the  reproduction  of  crayfishes  may  be 
enumerated  as  follows :  — 

(i)  Copulation,  during  which  the  spermatozoa  are  transferred 
from  the  vasa  deferentia  of  the  male  to  the  seminal  receptacle  of 
the  female;  (2)  egg-laying;  (3)  the  embryonic  development 
of  the  eggs;  (4)  hatching;  (5)  the  growth  of  the  young  cray- 
fishes. 

COPULATION.  —  Copulation  in  crayfishes,  in  most  cases,  takes 
place  in  September,  October,  or  November  of  the  first  year  of 


FIG.  109.  Male  crayfish  transferring  spermatozoa  to  the  seminal  receptacle 
of  the  female.     (From  Andrews  in  Am.  Nat.) 

their  lives,  that  is,  when  they  are  about  four  months  old.  A  sec- 
ond copulating  season  is  passed  through  at  the  end  of  the  second 
summer,  when  the  animals  are  about  seventeen  months  old,  and  a 
third  copulating  season  occurs  at  the  end  of  the  third  summer 
(179).  At  these  times  a  male  approaches  a  female,  grasps  her 
by  her  cephalic  appendages,  and,  after  a  struggle,  turns  her  over 
on  her  back.  He  then  stands  over  her  in  the  position  shown  in 
Figure  109,  and  transfers  spermatozoa  to  the  seminal  receptacle. 
During  this  process,  the  spermatozoa  flow  out  of  the  openings  of 
the  vasa  deferentia,  pass  along  the  grooves  on  the  first  abdominal 
appendages  of  the  male  (Fig.  101,  14),  and  enter  the  seminal  re- 
ceptacle (159).  Here  they  are  stored  during  the  winter.  The 
seminal  receptacle  is  a  cavity  in  a  fold  of  cuticle  lying  between  the 


THE  CRAYFISH  AND  ARTHROPODS  IN  GENERAL 


213 


sterna  of  the  segments  bearing   the  fourth  and  fifth  pairs  of 
walking  legs. 

EGG-LAYING  (159,  160).  — The  eggs  are  laid  at  night  during 
the  month  of  April.  First  the  ventral  side  of  the  abdomen  is 
thoroughly  cleaned  of  all  dirt  by  the  hooks  and  comb-like  bristles 
near  the  end  of  the  fifth  pair  of  walking  legs.  A  clear  slime  or 
glair  is  secreted  by  cement  glands  situated  chiefly  on  the  basal 
parts  of  the  uropods,  and  on  the  endopodites  of  the  other  abdomi- 
nal appendages.  This  milky  glair  gradually  covers  the  swim- 
merets.  The  female  then  lies  on  her  back,  and  an  apron-like 
film  of  glair  is  constructed  between  the  ends  of  the  uropods  and 
telson,  and  the  bases  of  the  second  pair  of  walking  legs  (Fig.  no). 


FIG.  no.  Female  crayfish  lying  on  back  laying  eggs.     (From  Andrews  ir 

Am.  Nat.} 

The  eggs  emerge  from  the  openings  of  the  oviducts  in  the  bases 
of  the  third  pair  of  walking  legs,  flow  posteriorly,  and  become 
attached  to  the  hairs  on  the  swimmerets  by  strings  of  a  substance 


214 


AN  INTRODUCTION  TO  ZOOLOGY 


no  doubt  secreted  by  the  cement  glands.  This  is  brought  about 
by  the  turning  of  the  animal  first  on  one  side  and  then  on  the 
other  a  number  of  times.  From  one  hundred  to  over  six  hundred 
greenish  eggs  are  laid  by  a  single  female,  depending  upon  the 
size  and  age  of  the  animal.  After  the  eggs  are  laid  the  crayfish 
rights  herself,  the  apron  of  glair  breaks  down,  and  the  abdomen 
is  extended  (Fig.  in). 


FIG.  in.  Female  aerating  eggs  by  raising  and  straightening  abdomen  and 
waving  swimmerets  back  and  forth.     (From  Andrews  in  Am.  Nat.} 

FERTILIZATION.  —  The  method  of  fertilization  has  never  been 
discovered.  It  is  supposed  that  as  the  eggs  are  laid  they  pass 
over  the  opening  of  the  seminal  receptacle,  and  are  then  pene- 
trated by  the  spermatozoa  which  were  placed  there  by  the  male 
the  preceding  autumn  (160). 

Embryology.  —  While  the  eggs  are  developing  they  are  pro- 
tected by  the  abdomen  of  the  female,  and  are  aerated  and  kept 
free  from  dirt  by  the  waving  of  the  swimmerets  back  and  forth 
(Fig.  in).  The  embryology  of  Cambarus  has  never  been  inves- 
tigated, but  it  probably  resembles  closely  the  development  of  the 
common  European  crayfish,  Astacus  flumatilis. 

The  fertilized  egg  of  Astacus  consists  of  a  large  number  of 
yolk  spheres  embedded  in  cytoplasm,  and  contains  a  nucleus 
composed  of  the  egg  nucleus  and  the  spermatozoon  combined. 
This  nucleus  proceeds  to  form  two  daughter  nuclei.  The  egg 
does  not  become  divided  into  two  parts  by  cell  walls,  but  the 
daughter  nuclei  separate  from  one  another  and  divide  again 


THE   CRAYFISH   AND   ARTHROPODS  IN   GENERAL        215 


(Fig.  112,  A).  This  division  continues  until  there  are  a  large 
number  of  nuclei  scattered  about  among  the  yolk  spheres.  Fi- 
nally all  of  the  nuclei  migrate  toward  the  periphery  of  the  egg  and 
come  to  lie  just  beneath  the  surface  (Fig.  112,  B).  Cell  walls 
then  appear,  dividing  the  egg  into  as  many  yolk  pyramids  as  there 
are  nuclei  (Fig.  112,  C).  The  outer  ends  of  these  yolk  pyramids 
containing  the  nuclei  are  now  cut  off  by  cross  walls,  the  result  being 
a  single  layer  of  cells,  the  blastoderm,  surrounding  the  entire  egg. 
The  side  walls  of  the  inner  portion  of  the  pyramids  fuse,  partially 
restoring  the  original  condition. 


FIG.  112.  Three  stages  in  the  de- 
velopment of  the  egg  of  the 
crayfish.  A,  a  few  cleavage 
nuclei  in  the  egg;  B,  nuclei 
arranged  near  the  surface ; 
C,  egg  divided  into  yolk 
pyramids.  (From  Korschelt 
and  Heider.  A  and  B,  after 
Morin  ;  C,  after  Reichenbach.) 

The  embryology  of  the  cray- 
fish is  so  complex  that  a  de- 
tailed account  will  not  be 
attempted  here.  It  will  suffice  to  make  a  superficial  examina- 
tion of  the  developing  embryo  with  the  aid  of  Figures  112-115. 
A.  thickening  of  the  blastoderm  on  one  side  of  the  egg  is  known 


2l6  AN  INTRODUCTION  TO  ZOOLOGY 

as  the  primitive  streak.  Five  other  thickenings  also  arise  in  this 
region,  a  pair  of  optic  lobes  (Fig.  113,  K)  a  pair  of  thoraco-abdominal 
plates  (TA.),  and  a  median  entodermal  plate  (ES.).  The  side 
on  which  these  thickenings  occur  will  become  the  ventral  surface 
of  the  adult. 

A  later  stage  (Fig.  114)  shows  a  median  outgrowth  just  back 
of  the  optic  lobes  which  will  become  the  upper  lip,  orlabrum  (/), 
and  pairs  of  rudimentary  antennules  (a,),  antenna  (a2)  and  man- 
dibles (m).  Between  the  antennae  is  a  depression,  which  will 
become  the  mouth.  An  opening  in  the  thoraco-abdominal  plate 
(TA.)  represents  the  future  anus  (A).  In  a  more  advanced 
embryo  (Fig.  115)  the  structures  already  mentioned  are  seen  in  a 
later  stage  of  development.  The  thoraco-abdominal  rudiment 
has  become  divided  into  a  number  of  segments,  from  each  of 
which  a  pair  of  appendages  arise  and  develop  into  those  of  the 
adult  crayfish  (171).  The  length  of  time  required  for  the  various 
stages  in  development  was  found  by  Andrews  to  be  as  follows: 
"  Cleavage  took  up  the  first  week,  the  beginning  of  an  embryo  the 
second  week,  to  progress  as  far  as  the  Nauplius  the  third  week 
and  more,  to  enlarge  the  embryo  over  one  half  of  the  egg  a  fourth 
week  and  more,  and  to  perfect  the  embryo  for  hatching  a  fifth  and 
sixth  week  or  more.  The  whole  egg  development  required  from 
five  to  eight  weeks  in  different  sets  of  eggs  under  different  tempera- 
ture "  (159,  pp.  189-190). 

HATCHING  (159,  163).  —  In  hatching,  the  egg  capsule  splits  and 
the  larva  emerges  head  foremost.  The  helpless  young  crayfish 
would  drop  away  from  the  mother  at  once  but  for  a  thread  extend- 
ing from  its  telson  to  the  inner  surface  of  the  egg  capsule  (Fig. 
1x6). 

Soon  the  larva  possesses  strength  enough  to  grasp  the  egg  string 
with  its  claws  (Fig.  116).  The  telson  thread  then  breaks.  After 
about  forty-eight  hours  the  larva  passes  into  a  second  stage.  This 
is  inaugurated  by  the  shedding  of  the  first  larval  cuticular  cover- 
ing, a  process  known  as  molting  or  ecdysis.  This  casting  off  of 
the  covering  of  the  body  is  not  peculiar  to  the  young,  but  occurs 


FIG.  113.  Surface  of  a  crayfish  egg  with  embryo  be- 
ginning to  form.  ES,  entodermal  plate  ;  K,  optic 
lobes ;  TA ,  thoraco-abdominal  plates.  (From 
Korschelt  and  Heider  after  Reichenbach.) 


FIG.  114.  Embryo  of  the  crayfish  in  the  Nauplius  stage. 
A,  rudiment  of  eye;  ai,  antennule ;  02,  antenna; 
G,  cerebral  ganglion  ;  m,  mandible  ;  /,  upper  lip  ; 
TA ,  thoraco-abdominal  plate  ;  A ,  anus.  (From 
Korschelt  and  Heider  after  Reichenbach.) 


FIG.  115.  Older  embryo  of  the  crayfish.  A,  eyes; 
ai,  antennules ;  a-i,  antenna ;  ab,  abdomen ;  /,  up- 
per lip;  w,  mandible;  w.n,  mx**,  first  and  second 
maxillae  ;  T,  telson  ;  /;,  /g,  thoracic  limbs  ;  h,  ts, 
maxillipedes  ;  /4,  /8,  walking  legs.  (From  Korschelt 
and  Heider  after  Reichenbach.) 


THE   CRAYFISH  AND   ARTHROPODS  IN   GENERAL      217 

in  adult  crayfishes  as  well  as  in  young  and  adults  of  many  other 
animals.  In  the  larval  crayfish  the  cuticle  of  the  first  stage  be- 
comes loosened  and  drops  off.  In  the  meantime  the  hypodermal 
cells  have  secreted  a  new  covering.  Ecdysis  is  necessary  before 
growth  can  proceed,  since  the  chitin  of  which  the  exoskeleton  is 
composed  does  not  allow  expansion.  In  adults  it  is  also  a  means 
of  getting  rid  of  an  old  worn-out  coat  and  acquiring  a  new  one. 

In  the  second  larval  stage  the  young  crayfish  is  again  supported 
immediately  after  casting  off  its  covering  by  a  thread  extending 
from  the  new  to  the  old  telson.  When  the  larva  becomes  strong 


FIG.  116.  Larva  of  crayfish  24  hours  after  hatching  ;  claws  fastened  to  stalk 
of  eggshell  and  abdomen  fastened  to  inside  of  shell.  (From  Andrews  in 
Am.  Nat.) 

enough,  it  grasps  the  old  larval  skin  or  swimmerets,  and  the  telson 
thread  drops  off.  The  duration  of  the  second  larval  stage  is 
about  six  days. 

No  telson  thread  is  present  after  the  molt  which  ushers  in  the 
third  larval  stage,  but  the  young  is  able  at  once  to  cling  to  the  old 
cuticle.  In  about  a  week  the  third  larvae  become  entirely  inde- 


218 


AN  INTRODUCTION  TO  ZOOLOGY 


TABLE  IX 

THE  DURATION  OF  THE  LARVAL  STAGES,  AND  THE  SIZE  AND  HABITS  OF  YOUNG 
CRAYFISHES   DURING   THE   FIRST    SUMMER 


STAGE 

DURATION 

SIZE 

HABIT 

ISt 

2  days 

4  mm. 

Attached  to  mother. 

2d 

6  days 

5  mm. 

Attached  to  mother. 

3d 

1  8  days 

8  mm. 

Associated         with 

mother     for     one 

week,  then  free. 

4th 

17  days 

12  mm. 

Free. 

5th 

5  days 

17  mm. 

Free. 

6th 

ii  days 

21  mm. 

Free. 

7th 

? 

29  mm. 

Free. 

End  of  summer 

4  months 

41  mm. 

Free. 

pendent  of  the  mother,  although  they  at  first  always  return  to  her 
when  separated.     From  this  time  on  the  larvae  shift  for  themselves, 

TABLE  X 

THE  AVERAGE  RATE  OF  GROWTH   OF  A  CRAYFISH    (CdmbarUS  affinis) 


AGE 

TIME  INTERVAL 

SIZE 

Just  hatched 

4  mm. 

End  of  ist  summer 

4  months 

41  mm. 

End  of  2d  summer 

1  6  months 

74  mm. 

End  of  3d  summer 

28  months 

90  mm. 

End  of  4th  summer 

40  months 

98  mm. 

growing  rapidly,  and  molting  at  least  four  more  times  during 
the  first  summer.  The  first  winter  no  growth  nor  molts  occur. 
There  are  four  or  five  molts  the  second  summer,  three  or  four  in 
the  third  summer,  and  perhaps  one  or  two  in  the  fourth  summer. 


THE   CRAYFISH  AND   ARTHROPODS  IN   GENERAL      219 


The  life  of  a  single  individual  extends  over  a  period  of  about  three 
or  more  years.  Table  IX  presents  the  main  facts  of  larval  de- 
velopment during  the  first  summer  (159).  Table  X  gives  the 
age  and  size  of  a  single  specimen  during  its  entire  life. 

Regeneration.  —  The  crayfish  and  many  other  Crustaceans 
have  the  power  of  regenerating  lost  parts,  but  to  a  much  more 
limited  extent  than  such  animals  as  Hydra  and  the  earthworm. 
Experiments  have  been  performed  upon  almost  every  one  of  the 
appendages  as  well  as  the  eye.  The  second  and  third  maxillipeds, 
the  walking  legs,  the  swimmerets,  and  the  eye  have  all  been 
injured  or  extirpated  at  various  times  and  subsequently  renewed 
the  lost  parts.  Many  species  of  crayfish  of  various  ages  have  been 
used  for  these  experiments.  The  growth  of  regenerated  tissue 
is  more  frequent  and  rapid  in  young  specimens  than  in  adults 
(184). 

The  new  structure  is  not  always  like  that  of  the  one  removed. 
For  example,  when  the  annulus  containing  the  sperm  receptacle 
of  an  adult  is  extirpated,  another  is  regenerated,  but,  although  this 
is  as  large  as  that  of  the  adult,  it  is  comparable  in  complexity  to 
that  of  an  early  larval 
stage  (161).  A  more 
remarkable  phenome- 
non is  the  regeneration 
of  an  apparently  func- 
tional (tactile)  an- 
tenna-like organ  in 
place  of  a  degenerate 
eye  which  was  removed 
from  the  blind  crayfish, 
Cambarus  pellucidus 
testii.  In  this  case  a 
non-functional  organ  FIG.  117.  Diagram  showing  antenna-like  organ 
was  replaced  by  a  func-  regenerated  in  place  of  an  eye  of  Palae- 

tional  one  of  a  differ-          mon'    (From  Morgan  after  HerbsL) 
ent  character  (190).     The  regeneration  of  a  new  part  which  differs 


r 


220  AN  INTRODUCTION  TO  ZOOLOGY 

from  the  part  removed  is  termed  heteromorphosis.  Figure  117 
shows  an  antenna  which  regenerated  in  place  of  an  eye  in  a  marine 
crustacean,  Pal&mon.  Instances  of  heteromorphosis  have  also 
been  recorded  in  experiments  on  other  animals  (176). 

Autotomy.  —  Perhaps  the  most  interesting  morphological 
structure  connected  with  the  regenerative  process  in  Cambarus 
is  the  definite  breaking  point  near  the  bases  of  the  walking  legs. 
If  the  chelae  are  injured,  they  are  broken  off  by  the  crayfish  at  the 
breaking  point.  The  other  walking  legs,  if  injured,  may  be  thrown 
off  at  the  free  joint  between  the  second  and  third  segments.  A 
new  leg,  as  large  as  the  one  lost,  develops  from  the  end  of  the 
stump  remaining.  This  breaking  off  of  the  legs  at  a  definite  point 
is  known  as  autotomy,  a  phenomenon  that  also  occurs  in  a  number 
of  other  animals.  The  leg  is  separated  along  the  breaking  point 
by  several  successive  muscular  contractions.  It  has  been  shown 
"  that  autotomy  is  not  due  to  a  weakness  at  the  breaking  point, 
but  to  a  reflex  action,  and  that  it  may  be  brought  about  by  a 
stimulation  of  the  thoracic  ganglion  as  well  as  by  a  stimulation 
of  the  nerve  of  the  leg  itself  "  (182,  p.  310).  Immediately  after 
the  leg  has  been  thrown  off,  a  membrane  of  ectoderm  cells  covers 
the  canal  through  which  the  nerve  and  blood  passed;  five  days 
later  regeneration  begins  by  an  outward  growth  of  the  ectoderm 
cells  which  lined  the  exoskeleton.  An  interesting  point  in  this 
new  growth  is  that  the  muscles  of  the  regenerated  part  are  prob- 
ably produced  by  ectoderm  cells,  whereas  in  the  embryonic  de- 
velopment of  the  crayfish  the  muscles  are  supposed  to  arise  from 
the  entoderm  (178). 

The  power  of  autotomy  is  of  advantage  to  the  crayfish,  since 
the  wound  closes  more  quickly  if  the  leg  is  lost  at  the  breaking 
point.  No  one  has  yet  offered  an  adequate  theory  to  account 
for  autotomy.  It  is  probably  "  a  process  that  the  animal  has 
acquired  in  connection  with  the  condition  under  which  it  lives, 
or  in  other  words,  an  adaptive  response  of  the  organism  to  its 
condition  of  life  "  (176,  p.  158). 

As  in  the  earthworm,  the  rate  of  regeneration  depends  upon  the 


THE  CRAYFISH  AND  ARTHROPODS  IN   GENERAL       221 

amount  of  tissue  removed.  If  one  chela  is  amputated,  a  new 
chela  regenerates  less  rapidly  than  if  both  chelae  and  some  of  the 
other  walking  legs  are  removed  (189). 

Behavior.  —  When  at  rest,  the  crayfish  usually  faces  the  en- 
trance to  its  place  of  concealment,  and  extends  its  antennae. 
It  is  thus  in  a  position  to  learn  the  nature  of  any  approaching 
object  without  being  detected.  Activity  at  this  time  is  reduced 
to  the  movements  of  a  few  of  the  appendages  and  the  gills;  the 
scaphognathites  of  the  second  maxillae  move  back  and  forth  baling 
water  out  of  the  forward  end  of  the  gill  chambers;  the  swimmer- 
ets  are  in  constant  motion  creating  a  current  of  water;  the  maxil- 
lipeds  are  likewise  kept  moving;  and  the  antennae  and  eye  stalks 
bend  from  place  to  place. 

Crayfishes  are  more  active  at  nightfall  and  at  daybreak  than 
during  the  remainder  of  the  day.  At  these  times  they  venture 
out  of  their  hiding  places  in  search  of  food,  their  movements 
being  apparently  all  utilitarian  and  not  for  spontaneous  play  or 
exercise  (167). 

LOCOMOTION.  —  Locomotion  is  effected  in  two  ways,  walking 
and  swimming.  Crayfishes  are  able  to  walk  in  any  direction, 
forward  usually,  but  also  sidewise,  obliquely,  or  backward.  In 
walking,  the  fourth  pair  of  legs  are  most  effective  and  bear  nearly 
all  of  the  weight  of  the  animal ;  the  fifth  pair  serve  as  props,  and 
to  push  the  body  forward;  the  second  and  third  pairs  are  less 
efficient  for  walking,  since  they  are  modified  to  serve  as  prehensile 
organs,  and  as  toilet  implements  (168).  Swimming  is  not  re- 
sorted to  under  ordinary  conditions,  but  only  when  the  animal  is 
frightened  or  shocked.  In  such  a  case  the  crayfish  extends  the 
abdomen,  spreads  out  the  uropods  and  telson,  and,  by  sudden 
contractions  of  the  bundles  of  flexor  abdominal  muscles,  bends  the 
abdomen  and  darts  backwards.  The  swimming  reaction  appar- 
ently is  not  voluntary,  but  is  almost  entirely  reflex  (168). 

EQUILIBRATION.  —  The  crayfish  either  at  rest  or  in  motion  is  in 
a  state  of  unstable  equilibrium,  and  must  maintain  its  body  in  the 
normal  position  by  its  own  efforts.  The  force  of  gravity  tends 


222  AN  INTRODUCTION  TO  ZOOLOGY 

to  turn  the  body  over.  From  a  large  number  of  experiments  it 
has  been  proven  that  the  statocysts  are  the  organs  of  equilibra- 
tion. The  structure  of  these  organs  is  described  on  page  208. 
The  contact  of  the  statoliths  with  the  statocyst  hairs  furnishes 
the  stimulus  which  causes  the  animal  to  maintain  an  upright  posi- 
tion. 

When  placed  on  its  back,  the  crayfish  has  some  difficulty  in 
righting  itself.  Two  methods  of  regaining  its  normal  position 
are  employed.  The  usual  method  is  that  of  raising  itself  on  one 
side  and  allowing  the  body  to  tip  over  by  the  force  of  gravity. 
The  second  method  is  that  of  contracting  the  flexor  abdominal 
muscles  which  causes  a  quick  backward  flop,  bringing  the  body 
right  side  up  (168).  In  general,  the  animals  "  right  themselves, 
when  placed  on  their  backs,  by  the  easiest  method;  and  this  is 
found  to  depend  usually  upon  the  relative  weight  of  the  two  sides 
of  the  body.  When  placed  upon  a  surf  ace  which  is  not  level,  they 
take  advantage,  after  a  few  experiences,  of  the  inclination  by  turn- 
ing toward  the  lower  side  "  (188,  p.  577). 

SENSES  AND  THEIR  LOCATION.  —  The  sense  of  touch  in  crayfishes 
is  perhaps  the  most  valuable,  since  it  aids  them  in  finding  food, 
avoiding  obstacles,  and  in  many  other  ways.  It  is  located  in 
specialized  hairs  on  various  parts  of  the  body  (164).  Vision  in 
crayfishes  is  probably  of  minor  importance,  since  the  compound 
eyes  are  almost  useless  in  recognizing  form,  and  are  of  real  value 
only  in  detecting  moving  objects.  No  reactions  to  sound  have 
ever  been  observed  in  crayfishes,  and  apparently  they  do  not 
hear.  "  The  reactions  formerly  attributed  to  audition  are  prob- 
ably due  to  tactile  reflexes  "  (181,  p.  244).  In  aquatic  animals 
it  is  so  difficult  to  distinguish  between  reactions  of  taste  and  smell 
that  these  senses  are  both  included  in  the  term  chemical  sense. 
The  end  organs  of  this  sense  are  distributed  all  over  the  body. 

REACTIONS  TO  STIMULI. — THTGMOTROPISM. — The  crayfish  "  is 
sensitive  to  touch  over  the  whole  surface  of  the  body,  but  espe- 
cially on  the  chelae  and  chelipedes,  the  mouth  parts,  the  ventral 
surface  of  the  abdomen,  and  the  edge  of  the  telson  "  (164,  p.  644). 


THE   CRAYFISH  AND   ARTHROPODS  IN   GENERAL      223 

The  antennae  are  usually  considered  the  special  organs  of  touch, 
but  experiments  seem  to  prove  that  they  are  not  so  sensitive  as 
other  parts  of  the  body.  The  tactile  hairs  are  plumed,  and  sup- 
plied by  a  single  nerve.  Positive  thigmotropism  is  exhibited  by 
crayfishes  to  a  marked  degree,  the  animals  seeking  to  place  their 
bodies  in  contact  with  a  solid  object,  if  possible.  The  normal 
position  of  the  crayfish  when  at  rest  under  a  stone  is  such  as  to 
bring  its  sides  or  dorsal  surface  in  contact  with  the  walls  of  its 
hiding  place.  Thigmotropism,  no  doubt,  is  of  distinct  advan- 
tage, since  it  forces  the  animal  into  a  place  of  safety. 

PHOTOTROPISM.  —  Light  of  various  intensities  in  the  majority 
of  cases  causes  the  crayfish  to  retreat,  i.e.  to  show  negative 
phototropism.  Individuals  prefer  colored  lights  to  white,  having 
a  special  liking  for  red.  Negative  reactions  to  light  play  an 
important  role  in  the  animal's  life,  since  they  influence  it  to  seek  a 
dark  place  where  it  is  concealed  from  its  enemies. 

CHEMOTROPISM.  —  The  reactions  of  the  crayfish  to  food  are  due 
in  part  to  a  chemical  sense.  Smooth  hairs,  with  nerve  bundles 
within  them,  are  probably  chemical,  and,  since  "  The  animals 
react  to  chemical  stimulation  on  any  part  of  the  body  ...  we 
must  assume  that  there  are  chemical  sense  organs  all  over  the 
body  "  (165,  p.  325).  The  anterior  appendages,  however,  are 
the  most  sensitive,  especially  the  outer  ramus  of  the  antennule. 
Positive  reactions  result  from  the  application  of  food  substances. 
For  example,  if  meat  juice  is  placed  in  the  water  near  an  animal, 
the  antennae  move  slightly  and  the  mouth  parts  perform  vigorous 
chewing  movements.  The  meat  causes  "  general  restlessness 
and  vague  movements  toward  the  source  of  the  stimulation,  but 
the  animals  seem  to  depend  chiefly  on  touch  for  the  accurate 
localization  of  food  "  (165,  p.  326).  Acids,  salts,  sugar,  and  other 
chemicals  produce  a  sort  of  negative  reaction  indicated  by  scratch- 
ing the  carapace,  rubbing  the  chelae,  or  pulling  at  the  part  stimu- 
lated. 

HABIT  FORMATION.  —  It  has  been  shown  by  certain  simple 
experiments  that  crayfishes  are  able  to  learn  habits  and  to  modify 


224 


AN  INTRODUCTION  TO  ZOOLOGY 


them.  They  learn  by  experience,  and  modify  their  behavior 
slowly  or  quickly,  depending  upon  their  familiarity  with  the 
situation.  The  following  experiments  prove  the  above  state- 
ments. Crayfishes,  when  placed  in  compartment  T  of  a  labyrinth 
like  that  shown  in  Figure  118,  will,  on  attempting  to  escape, 
pass  on  one  side  or  the  other  of  the  partition  P.  A  transparent 
glass  screen  G  prevents  their  exit  from  the  right  side  of  the  parti- 
tion; the  other  side,  however,  being  left  open,  allows  a  free  pas- 
sage. It  was  found  by  Yerkes  and  Huggins  (188)  that  after 
sixty  trials,  crayfishes  that  at  the  beginning  chose  the  closed 
passage  50  per  cent  of  the  time,  learned  to  avoid  that  side,  and  in 

__^      go  per  cent  of  the 

trials  chose  the 
open  exit.  When 
the  glass  plate 
was  then  removed 
from  the  right  of 
the  partition  and 
placed  on  the 
other  side,  the 
crayfishes  were 
confused  at  first, 
but  learned  the 
new  habit  of  es- 
caping at  the 
right,  open  side 


T 

A 


FIG.  118.  Labyrinth  used  in  experiments  on  the  cray- 
fish. T,  compartment  from  which  animal  was 
started;  P,  partition  at  exit;  G,  glass  plate 
closing  one  exit.  (From  Washburn  after  Yerkes 
and  Huggins.) 


more  quickly  than  before.  The  "  chief  factors  in  the  formation 
of  such  habits  are  the  chemical  sense  (probably  both  smell  and 
taste),  touch,  sight,  and  the  muscular  sensations  resulting  from 
the  direction  of  turning.  The  animals  are  able  to  learn  a  path 
when  the  possibility  of  following  a  scent  is  excluded  "  (188,  p. 
577).  Professor  S.  J.  Holmes  writes  that  he  has  trained  cray- 
fishes to  come  to  him  for  food. 


THE   CRAYFISH  AND   ARTHROPODS  IN   GENERAL        225 


2.  ARTHROPODS  IN  GENERAL 

All  the  segmented  animals  bearing  jointed 
appendages  belonging  to  the  Phylum  Arthro- 
poda  may  be  grouped  in  five  classes :  — 

Class  I.  Crustacea  (Crayfish,  Crabs,  Bar- 
nacles, Water-fleas,  etc.). 

Class  II.     Onychophora  (Peripatus). 

Class  III.  Myriapoda  (Centipedes  and 
Millipedes). 

Class  IV.     Insecta  (Insects). 

Class  V.  Arachnida  (Spiders,  Mites,  Scor- 
pions, King  crabs,  etc.). 

These  five  classes  are  often  divided  for 
convenience  into  two  large  groups,  the  Bran- 
chiata  containing  Class  I,  and  the  Tra- 
cheata,  Classes  II-V.  Members  of  the  former 
division  are  mainly  aquatic  and  breathe  by 
means  of  gills.  The  Tracheata  are  in  most 
cases  terrestrial,  and  obtain  oxygen  from  air 
taken  into  a  complex  system  of  tubules,  called 
tracheae,  which  ramify  to  all  parts  of  the 
body. 

Arthropods  are  supposed  to  be  closely 
related  to  Annelids.  The  members  of  both 
phyla  are  segmented,  bilaterally  symmetrical, 
and  triploblastic,  with  a  dorsal  brain  in  the 
head  and  a  ventral  nerve  cord,  a  dorsal  heart, 
and  an  external  chitinous  covering.  The 
following  differences  may  be  pointed  out : 
Annelids  possess  a  large  number  of  similar 
segments;  Arthropods,  in  most  cases,  a 
limited  number  of  much  modified  segments: 
the  former  have  segmentally  arranged 
nephridia;  the  excretory  organs  of  the  latter, 


FIG.  119.  Ventral 
view  of  male  Cy- 
clops, i,  anten- 
nule ;  2,  anten- 
na ;  3,  mandible ; 

4,  ist    maxilla; 

5,  two    halves 
of    2d   maxilla; 
6-p,    i  s  t  -  4  t  h 
thoracic  limbs ; 
10,  eye;  n,  bris- 
tles  near   male 
genital  opening; 
12,  caudal  fork ; 
73,  mouth ;    14, 
copula  connect- 
ing  pairs  of 
limbs.        (From 
Shipley  and 
MacBride.) 


226 


AN   INTRODUCTION  TO  ZOOLOGY 


FIG.  1 20.  Peripatus  capensis.     (From  Shipley  and  MacBride  after  Sedgwick.) 

with  the  exception  of  Peripatus,  are  not  segmentally  arranged; 
Annelids  have  a  well  developed  ccelom; 
in  arthropods  the  ccelom  is  restricted 
to  the  cavities  of  the  excretory  and 
reproductive  organs. 

The  Crustacea  are  divided  into  two 
subclasses,  the  Entomostraca  and  the 
Malacostraca.  The  Entomostraca  in- 
cludes most  of  the  small  simple  species 
(Fig.  119).  These  have  a  variable 
number  of  segments;  no  gastric  mill 
in  the  stomach;  and  in  many  species 
hatch  as  a  larval  form  called  a  Nauplius 
(Fig.  126,  A).  The  Malacostraca  are 
usually  large.  They  have  a  definite 
number  of  segments  —  five  in  the  head, 
eight  in  the  thorax,  and  six  in  the 
abdomen;  the  stomach  contains  a  gas- 
tric mill;  and  the  Nauplius  stage  is 
usually  passed  through  within  the  egg 
before  hatching.  The  crayfish  belongs 
in  this  subclass. 

Class  Onychophora  contains  only  a 
few  annelid-like  animals,  the  best  known 

FIG.  121.  Dorsal  view  of  a  being  Peripatus  capensis  (Fig.  120). 
centipede.  /,  antenna;  Segmentally  arranged  nephridia,  stump- 
2,  poison  claw.  (From  like  legs,  simple  eyes,  and  tracheae  are 
Shipley  and  Mac-  some  of  ^Q  organs  possessed  by  mem- 
bers of  this  class. 


Bride.) 


THE  CRAYFISH  AND  ARTHROPODS  IN  GENERAL 


227 


III.  MYRIAPODA 


FIG.  122.  A  millipede.     (From  Shipley  and  MacBride  after  Koch.) 

The  myriopods  are  also  annelid-like  in  appearance.     The  two 
principal  orders  are  the  Chilopoda  and  Diplopoda.     The  Chilo- 
poda  or  centipedes  (Fig.  121)  are  flattened  dorso-ventrally,  have 
one  pair   of   legs  at- 
tached  to   every  seg- 
ment    back     of     the 
head,  and    possess   a 
pair  of   poison  claws 
attached   to  the  first 
segment.     The  Diplo- 
poda    or     Millipedes  ,.  CR 
(Fig.    122)  are   cylin- 
drical, have  two  pairs 
of    legs    attached    to 
every    segment    back 
of     the     fourth,    and 
lack  poison  claws. 

The  insects  consti- 
tute about  four  fifths 
of  all  the  species  of 
animals.  The  num- 
ber of  individuals  is 
even  more  remarka- 
ble. The  body  of  an 
insect  is  divided  into 
head,  thorax,  and  ab-  FlG.  I2,t  Diagr'am  to  show  the  affinities  of 
domen.  The  thorax  arthropods. 


PRIMITIVE  ARTHROPODS 


228  AN  INTRODUCTION  TO  ZOOLOGY 

bears  three  pairs  of  legs,  and,  with  a  few  exceptions,  one  or 
two  pairs  of  wings.  The  abdomen  has  several  or  no  append- 
ages. The  honeybee  described  in  Chapter  XII  has  been  selected 
as  a  type  of  this  class. 

The  Class  Arachnida  includes  animals  which  have  the  head 
and  thorax  fused  into  a  cephalo-thorax.  They  possess  four  pairs 
of  legs,  an  abdomen  usually  without  appendages,  and  a  heart  in 
the  dorsal  part  of  the  abdomen.  Such  diverse  animals  as  the 
spider,  scorpion,  mite,  and  King  crab  are  placed  together  in  this 
class.  The  probable  affinities  of  the  various  classes  of  arthro- 
pods are  shown  in  Figure  123. 

The  Biogenetic  Law.  —  In  concluding  this  chapter  we  wish 
to  refer  to  a  law  which  has  commanded  the  attention  of 
zoologists  for  almost  a  century;  namely,  the  law  of  bio- 
genesis, also  known  as  the  recapitulation  theory.  Organic  evo- 
lution, that  is,  the  evolution  of  one  organism  from  another, 
is  accepted  as  an  established  fact  by  practially  all  zoologists 
at  the  present  time.  This  fact  of  organic  evolution  is 
expressed  in  the  diagram  of  the  principal  phyla  of  the 
animal  kingdom  on  page  7,  where  the  various  groups  appear 
to  be  derived  from  a  central  stem  which  represents  a 
series  of  ancestral  forms.  From  an  examination  of  the  phyla 
represented  in  our  diagram  we  gain  the  fact  that  the  members  of 
each  group  are  more  complex  than  those  of  the  group  just  beneath 
them  on  the  page.  Evolutionists  do  not  claim  that  the  more 
complex  forms  have  evolved  directly  from  the  simpler  animals, 
but  that  their  ancestors  were  related.  Beginning  with  the  sim- 
plest animals  we  find  that  a  single  cell  performs  all  the  necessary 
processes  of  life,  e.g.  Ameba.  Within  the  lowest  phylum,  the 
Protozoa,  there  are  animals  consisting  of  a  number  of  cells  more 
or  less  intimately  bound  together  into  a  hollow  spherical  colony, 
e.g.  Volwx.  Passing  to  the  next  higher  group  of  organisms  we  are 
introduced  to  animals  that  possess  two  layers  of  cells,  surrounding 
a  single  cavity,  e.g.  Hydra.  All  animals  above  the  Ccelenterates 
have  three  layers  of  cells  forming  their  body  walls,  i.e.  are  triple- 


THE  CRAYFISH  AND   ARTHROPODS  IN   GENERAL        229 

blastic.  Four  stages  in  the  evolution  of  animals  are  represented 
in  the  groups  just  mentioned  —  (i)  the  single  cell,  (2)  a 
ball  of  cells,  (3)  a  two-layered  sac,  and  (4)  a  three-layered 
organism. 

Early  in  the  past  century  it  was  noticed  that  these  stages 
correspond  to  the  early  stages  in  the  embryology  of  the 
Metazoa;  in  other  words,  that  the  development  of  the 
individual  recapitulates  the  stages  in  the  evolution  of  the  race, 
or  ontogeny  recapitulates  phylogeny.  These  stages  contrasted 
appear  as  follows :  — 

Phylogenetic  Stage  Ontogenetic  Stage 

(1)  single-celled  animal  egg  cell 

(2)  ball  of  cells  blastula. 

(3)  two-layered  sac  gastrula. 

(4)  triploblastic  animal  three-layered  embryo. 

Later  other  zoologists  became  interested  in  the  recapitulation 
theory,  and  enlarged  upon  it.  Of  these  Fritz  Muller  and  Ernst 
Haeckel  are  especial- 
ly worthy  of  men- 
tion. The  latter 
expressed  the  facts  Xj 
as  he  saw  them  in  &' . 
his  "  fundamental 
law  of  biogenesis." 
The  ancestor  of  the 
many-celled  animals  FlG- I24-  Larva  of  lobster  in  Af 3-5/5  stage.  (From 

was     conceived    by 

him  as  a  two-layered  sac  something  like  a  gastrula,  which  he 
called  a  Gastrcea.  The  Ccelenterates  were  considered  to  be 
gastraea  slightly  modified. 

Fritz  Muller  derived  strong  arguments  in  favor  of  biogenesis 
from  a  study  of  certain  Crustacea  belonging  to  the  Malacostraca. 
Many  members  of  this  group  do  not  emerge  from  the  egg  so 


23° 


AN   INTRODUCTION   TO   ZOOLOGY 


FIG.  125.  Mysis  oculala.     (From  Sedgwick  after  Sars.) 


nearly  like  the  adult  as  does  the 
crayfish.      The  lobster,    for  ex- 

FIG.  126.  Two  stages  in  the  develop- 
ment of  the  shrimp,  Pcnaus.  A, 
Nauplius  stage ;  B,  Protozoaa  stage. 
(From  Sedgwick  after  Fritz 
Miiller.) 


B 


THE  CRAYFISH  AND  ARTHROPODS  IN  GENERAL 


231 


ample,  upon  hatching  (Fig.  124)  resembles  a  less  specialized 
prawn-like  crustacean,  called  My  sis  (Fig.  125),  and  is  said  to  be 
in  the  My  sis  stage.  The  shrimp,  Penceus,  passes  through  a 
number  of  interesting  stages  before  the  adult  condition  is 


FIG.  127.  Two  later  stages  in  the  development  of  Penaus.    A,  Zoaa  stage ; 
B,  My  sis  stage.     (From  Korschelt  and  Heider  after  Glaus.) 

attained.  It  hatches  as  a  larva,  termed  a  Nauplius  (Fig.  126,  A), 
possessing  a  frontal  eye  and  three  pairs  of  appendages  (A',  A" , 
Mdf.);  this  Nauplius  molts  and  grows  into  a  Protozo&a  stage 
(Fig.  126,  B)  which  bears  three  more  pairs  of  appendages  and 
the  rudiments  of  segments  III-VIII.  The  Protozoaea  stage 


232 


AN  INTRODUCTION   TO  ZOOLOGY 


grows  into  the  Zo<za  stage  (Fig.  127,  A).  The  cephalothorax 
and  abdomen  are  distinct  at  this  time ;  eight  pairs  of  appendages  are 
present  (I-VIII)  and  six  more  are  developing  (ai-fle).  TheZoaea 
grows  and  molts  and  becomes  a  My  sis  (Fig.  127,  B)  with  eight 
pairs  of  appendages  (I-VIII)  on  the  cephalothorax.  Finally  the 
Mysis  passes  into  the  adult  shrimp,  which  possesses  the 
characteristic  number  of  appendages  (I-XIX)  each  modified 
to  perform  its  particular  function.  The  Nauplius  of  Pen&us 
resembles  the  larvae  of  many  simple  crustaceans;  the  Zoaa  is 
somewhat  similar  to  the  condition  of  an  adult  Cyclops  (Fig.  119); 
the  Mysis  is  like  the  adult  Mysis  (Fig.  125) ;  and  finally  the  adult 
Penceus  is  more  specialized  than  any  of  its  larval  stages,  and  be- 
longs among  the  higher  Crustacea.  The  above  facts  have  con- 
vinced some  zoologists  that  Pen&us  recapitulates  in  its  larval 
development  the  progress  of  the  race;  that  the  lobster  has  lost 
many  of  these  stages,  retaining  only  the  Mysis;  and  that  the  cray- 
fish hatches  in  practically  the  adult  condition.  Tho  Nauplius 
stage  of  the  latter  is  supposed  to  be  represented  by  a  certain 
embryonic  phase  (Fig.  114). 

The  law  of  biogenesis  should  not  be  taken  too  seriously,  since 
it  has  been  criticized  severely  by  many  prominent-  zoologists, 
but  it  has  furnished  an  hypothesis,  which  has  concentrated  the 
attention  of  scientists  upon  fundamental  embryological  processes, 
and  has  therefore  had  a  great  influence  upon  zoological  progress 
(175,  177,  187). 


CHAPTER  XII 
THE  HONEYBEE  AND  BEES  IN  GENERAL 

i.  THE  HONEYBEE 
(Apis  melliftca  Linn.) 

THE  honeybee  has  been  selected  as  a  type  of  the  Class  In- 
secta,  of  the  Phylum  Arthropoda,  because  of  its  wonderful 
adaptations  to  its  environment;  its  complex  social  life;  its  eco- 
nomic value  to  man ;  and  the  ease  with  which  it  may  be  obtained 
for  dissection,  or  studied  alive  in  the  laboratory.  Honeybees 
have  been  made  the  objects  of  investigation  by  naturalists  for 
over  two  thousand  years,  and  the  habits  of  no  other  insect  are 
better  known.  They  live  in  colonies  in  which  there  are  three 
kinds  of  individuals  —  workers,  drones,  and  a  queen.  An  average 
colony  contains  in  the  summer  about  sixty  thousand  worker  bees, 
a  few  hundred  drones,  and  a  single  queen.  The  greatest  number 
will  be  found  in  a  hive  during  the  honey-gathering  season.  In 
the  winter  the  number  diminishes  by  several  thousands.  No 
drones  are  members  of  the  colony  at  this  time.  The  appearance 
and  functions  of  the  different  kinds  of  individuals  found  in  one 
hive  are  as  follows. 

The  Queen. — The  queen  (Fig.  128)  is  the  most  important 
member  of  the  hive,  since  normally  upon  her  and  her  alone  falls 
the  duty  of  reproduction.  She  lays  all  of  the  eggs,  and  is  the 
mother  of  every  worker  and  drone  in  the  colony.  She  lives  for 
three  years  or  more,  receiving  during  this  time  the  support  of  her 
many  offspring.  The  queen  may  be  distinguished  from  the 
worker  by  the  greater  length  of  her  abdomen  and  the  absence  of 
a  pollen  basket  on  the  tibia  of  her  hind  legs. 

233 


234  AN  INTRODUCTION  TO  ZOOLOGY 

The  Drone.  — The  drone  (Fig.  128)  is  the  male  bee;  he  lives 
in  idleness  upon  the  food  gathered  by  the  workers.  His  function, 
however,  is  an  important  one;  namely,  to  mate  with  the  queen. 
How  this  is  accomplished  will  be  described  later.  The  body  of 
the  drone  is  heavy  and  broad,  and  the  hind  legs  possess  no  pollen 
basket.  His  eyes  are  considerably  larger  than  those  of  either  the 
queen  or  worker. 

The  Worker.  —  The  worker  (Fig.  128)  is  a  sexually  undeveloped 
female.  Under  normal  conditions  it  lays  no  eggs,  but  is  kept 


9 

a  ma  • 

9 

Worker  Queen  Drone 

FIG.  128.  Honeybee,  Apis  mcllijica.      (From  Shipley  and  MacBride.) 

busy  gathering  honey,  pollen,  and  propolis,  or  "  bee-glue,"  carry- 
ing water,  secreting  wax,  building  comb,  preparing  food,  nursing 
the  young,  and  cleaning  and  defending  the  hive.  The  workers 
are  smaller  than  either  queen  or  drone;  they  are  the  bees  usually 
seen  hovering  about  flowers.  The  anatomical  description  that 
follows  concerns  the  worker  unless  otherwise  stated. 

Anatomy  and  Physiology.  —  EXOSKELETON.  —  The  body  of  the 
bee  is  completely  covered  with  a  skin  or  cuticle,  consisting  of  a 
very  thin  layer  of  chitin  which  is  secreted  by  the  hypodermal  cells 
lying  just  beneath  it.  This  chitin  protects  the  insect  from  injury 
and  gives  the  body  strength.  During  the  young  stages  the  exo- 
skeleton  of  inelastic  chitin  is  cast  off  at  intervals,  allowing  the 
body  to  expand. 

REGIONS  OF  THE  BODY. — Three  distinct  regions  may  be  recog- 
nized in  the  body  of  the  bee  —  the  head,  thorax,  and  abdomen. 
Each  division  consists  of  a  definite  number  of  segments  more  or 
less  intimately  fused  together. 


FIG.  129.  Head  of  worker  honeybee,  a,  antenna;  b,  bouton;  g,  cpiphar- 
ynx  ;  m,  mandible  ;  mx.,  maxilla  ;  mxp.,  maxillary  palpus  ;  /,  hypo- 
pharynx  ;  lp.,  labial  palpus.  (From  Packard  after  Cheshire.) 


THE  HONEYBEE  AND   BEES  IN  GENERAL  235 

THE  HEAD  AND  ITS  APPENDAGES  (Fig.  129).  —  The  segments 
comprising  the  head  can  only  be  distinguished  with  any  degree  of 
accuracy  in  the  embryo,  and  even  there  their  number  is  not 
certainly  established.  There  are  probably  six.  A  large  part  of 
the  head  is  occupied  by  the  compound  eyes,  one  on  either  side. 
These  differ  in  size  in  queen,  worker,  and  drone,  but  are  promi- 
nent in  all;  they  will  be  described  in  detail,  later.  Besides  these 
there  are  three  simple  eyes,  the  ocelli,  arranged  in  a  triangle  al- 
most directly  on  top  of  the  head  in  the  queen  and  worker,  and  in 
front  just  above  the  antennae  in  the  drone.  Projecting  from  the 
front  of  the  head  are  two  feelers  or  antenna  (Fig.  129,  a)  which 
serve  as  organs  of  special  sense. 

The  MOUTH  PARTS  consist  of  a  labrum  or  upper  lip,  the  epipharynx 
(Fig.  129,  g),  a  pair  of  mandibles  (m),  two  maxillae  (mx),  and  a 
labium  or  under  lip  (I,  lp.).  The  labrum  is  joined  to  a  dome- 
shaped  part  of  the  skull,  called  the  clypeus,  which  lies  just  above  it. 
From  beneath  the  labrum  projects  the  fleshy  epipharynx  (g); 
this  is  an  organ  of  taste.  The  mandibles,  or  jaws,  are  situated 
one  on  either  side  of  the  labrum;  they  are  notched  in  the  queen 
and  drone,  but  smooth  in  the  worker.  The  latter  makes  use  of 
them  in  building  honeycomb.  The  labium  is  a  complicated 
median  structure  extending  downward  from  beneath  the  labrum. 
It  is  joined  to  the  back  of  the  head  by  a  triangular  piece,  the  sub- 
mentum.  Next  to  this  is  a  chitinous,  muscle-filled  piece,  the 
mentum,  beyond  which  is  the  ligula,  or  tongue  (/) ,  with  one  labial 
palpus  (lp.)  on  each  side.  The  ligula  may  be  drawn  in  or  extended. 
It  is  long  and  flexible,  with  a  spoon  or  bouton  (b)  at  the  end.  Hairs 
of  various  kinds  are  arranged  upon  it  in  regular  rows;  these  are 
used  for  gathering  nectar,  and  as  organs  of  touch  and  taste. 

The  maxillce  (Fig.  129,  mx.),  or  lower  jaws,  fit  over  the  mentum 
on  either  side.  Along  their  front  edges  are  rows  of  stiff  hairs. 
Maxillary  palpi  (mxp.)  are  also  present.  Nectar  is  collected  in 
the  following  manner.  The  maxillae  and  the  labial  palpi  form  a 
tube  in  the  center  of  whrch  the  tongue  moves  backward  and  for- 
ward. When  the  epipharynx  is  lowered,  a  passage  is  completed 


230 


AN  INTRODUCTION  TO  ZOOLOGY 


into  the  oesophagus.  The  nectar  is  first  collected  by  the  hairs  on 
the  ligula;  it  is  then  forced  upward  by  the  pressing  together  of 
the  maxillae  and  labial  palpi. 

THE  THORAX  AND  ITS  APPENDAGES.  —  The  thorax  consists  of 
three  segments,  each  of  which  bears  a  pair  of  legs.     The  anterior 

segment  is  known  as  the  prothorax, 
the  middle  segment  as  the  mesotho- 
rax,  and  the  posterior  segment,  as 
the  metathorax.  The  mesothorax 
and  metathorax  each  support  a  pair 
of  wings.  The  segments  of  the 
thorax  are  comparatively  large, 
since  they  contain  the  largest  and 
most  important  muscles  of  the 
body.  Externally  the  thorax  is 
covered  with  flexible  branched 
hairs,  which  are  of  use  in  gathering 
pollen  (Fig.  130). 

LEGS.  —  Perhaps   the   most   in- 
teresting structures  of  the  honey- 
bee are  the  legs  of  the  worker  (Fig. 
131).     They    are    wonderfully 
adapted  for  the  wrork  which  they 
FIG.  130.  A,  worker   honeybee  perform.     The  parts  of   a   typical 
with  pollen  basket  full;   B     insect  j         mmi        them    -n    Qrder 
part  of  mesothoracic  leg  with 

branched  hairs  and  pollen  beginning  at  the  proximal  end,  are 
grains;  C,  one  hair  bearing  the  coxa  (c),  trochanter  (tr.),  femur 
pollen  grains.  (From  the  (J)f  tibia  (ti.),  and  five-jointed 
Cambridge  Natural  His-  iarsus  (A 

tory'}  The  prothoracic  legs  (Fig.  131,  C) 

possess  the  following  useful  structures.  The  femur  (/)  and  tibia 
(ti.)  are  clothed  with  branched  hairs  for  gathering  pollen.  Extend- 
ing on  one  side  from  the  distal  end  of  the  tibia  are  a  number  of 
curved  bristles,  the  pollen  brush  (b  in  C  and  E),  which  are  used  to 
brush  up  the  pollen  loosened  by  the  coarser  spines ;  on  the  other  side 


FIG.  131.  Legs  of  worker  bee.  (After  Cheshire.)  A,  outer  side  of  meta- 
thoracic  leg:  p,  metatarsus ;  /,  tarsus ;  //.,  tibia.  B,  inner  side  of 
metathoracic  leg:  c,  coxa  ;  p,  metatarsus  ;  /,  tarsus  ;  ti.,  tibia;  tr.,  tro- 
chanter;  wp.,  wax  pinchers.  C,  prothoracic  leg:  b,  pollen  brush;  eb., 
eye  brush ;  p,  metatarsus ;  t,  tarsus ;  ti.,  tibia ;  v,  velum.  D,  meso- 
thoracic  leg:  lettering  as  in  C  ;  s,  pollen  spur.  E,  joint  of  prothoracic 
leg ;  lettering  as  in  C.  F,  teeth  of  antenna  comb.  G,  cross  section  of 
tibia  through  pollen  basket ;  fa.,  pollen ;  h,  holding  hairs ;  n,  nerve. 
H,  antenna  in  process  of  cleaning ;  a,  antenna ;  c,  antenna  comb ;  / 
section  of  leg  ;  s,  scraping  edge  of  v,  velum.  (From  Root.) 


THE  HONEYBEE  AND  BEES  IN  GENERAL 


237 


is  a  flattened  movable  spine,  the  velum  (v  in  C  and  E)  which  fits 
over  a  curved  indentation  in  the  first  tarsal  joint  or  metatarsus 
(p  in  C).  This  entire  structure  is  called  the  antenna  cleaner  and 
the  row  of  teeth  (F)  which  lines  the  indentation  is  known  as  the 
antenna  comb.  Figure  131,  H,  shows  in  section  how  the  antenna 
(a)  is  cleaned  by  being  pulled  between  the  teeth  (c)  on  the  meta- 
tarsus (/)  and  the  edge  (s)  of 
the  velum  (v).  On  the  front  of 
the  metatarsus  is  a  row  of  spines 
(eb.  in  C)  called  the  eye  brush, 
which  is  used  to  brush  out  any 
pollen  or  foreign  particles  lodged 
among  the  hairs  on  the  com- 
pound eyes.  The  last  tarsal  joint 
of  every  leg  (Fig.  132)  bears  a 
pair  of  notched  claws  (an.)  which 
enable  the  bee  to  obtain  a  foot- 
hold on  rough  surfaces.  Between 
the  claws  is  a  fleshy  glandular 
lobule,  the  pulvillus  (pv.)  whose 
sticky  secretion  makes  it  possible 
for  the  bee  to  cling  to  smooth 
objects.  Tactile  hairs  are  also  FlG- 
present  (Jh.). 

The  middle,  or  mesothoracic  legs 
(Fig.  131,  D)  are  provided  with 
a  pollen  brush  (b),  but,  instead  of  an  antenna  cleaner,  a  spur  (s) 
is  present  at  the  distal  end  of  the  tibia.  This  spur  is  used  to  pry 
the  pollen  out  of  the  pollen  baskets  on  the  third  pair  of  legs,  and 
to  clean  the  wings. 

The  metathoracic  legs  (Fig.  131,  A  and  B)  possess  three  very 
remarkable  structures,  the  pollen  basket,  the  wax  pinchers 
(wp.  in  B),  and  the  pollen  combs  (at  p  in  B).  The  pollen  basket 
consists  of  a  concavity  in  the  outer  surface  of  the  tibia  with  rows 
of  curved  bristles  along  the  edges  (ti.  in  A).  By  storing  pollen 


132.  Foot  of  honeybee,  an., 
claw  ;  fh.,  tactile  hairs  ;  PV.,  pul- 
villus ;  /,  tarsal  joints.  (From 
Packard  after  Cheshire.) 


238  AN  INTRODUCTION  TO  ZOOLOGY 

in  this  basketlike  structure,  it  is  possible  for  the  bee  to  spend  more 
time  in  the  field,  and  to  carry  a  larger  load  at  each  trip.  The 
pollen  basket  in  cross  section  is  shown  in  Figure  131,  G.  The 
pollen  combs  (at  p  in  B)  serve  to  fill  the  baskets  by  combing  out 
the  pollen,  which  has  become  entangled  in  the  hairs  on  the  thorax, 
and  transferring  it  to  the  concavity  in  the  tibia  of  the  opposite 
leg.  At  the  distal  end  of  the  tibia  is  a  row  of  wide  spines;  these 
are  opposed  by  a  smooth  plate  on  the  proximal  end  of  the  meta- 
tarsus. The  term  wax  pinchers  (wp.  in  B)  has  been  applied  to 
these  structures,  since  they  are  used  to  remove  the  wax  plates 
from  the  abdomen  of  the  worker. 

WALKING.  —  In  walking  three  legs  are  extended  at  one  time, 
the  other  three  supporting  the  body  like  a  tripod.  The  foreleg 
pulls  the  body  forward,  the  middle  leg  maintains  equilibrium, 
and  the  hind  leg  pushes  the  insect  forward.  The  details  of  walk- 
ing in  insects  are  very  complicated. 

WINGS.  —  Each  of  the  first  two  thoracic  segments  bears  a  pair 
of  membranous  wings.  When  at  rest  the  wings  He  horizontally 
over  the  abdomen,  but  when  flying  are  widely  spread.  The  wings 
may  be  described  as  transparent  membranes  supported  by  hollow 
ribs  called  nerves  or  veins.  The  pair  of  wings  on  one  side  of  the 
body  may  be  joined  together  by  a  row  of  hooklels  on  the  anterior 
margin  of  the  hind  wing,  wrhich  are  inserted  into  a  troughlike  fold 
in  the  posterior  margin  of  the  fore  wing. 

FLIGHT.  —  When  flying  the  wings  act  as  inclined  planes,  and 
locomotion  forward  is  attained  by  both  up  and  down  strokes, 
the  tips  of  the  wings  moving  in  a  curve  shaped  like  a  figure  8. 
Motion  backward,  or  a  sudden  stop,  may  be  accomplished  by 
changing  the  inclination  of  the  plane  of  oscillation. 

THE  ABDOMEN  AND  ITS  APPENDAGE.  —  The  abdomen  is  made  up 
of  a  series  of  six  visible  segments  or  rings  of  chitin  inclosing  a  large 
part  of  the  alimentary  canal,  nervous  system,  reproductive  organs, 
etc.  Each  ring  has  a  dorsal  plate,  the  tergum,  and  a  ventral  plate, 
the  sternum.  Thin  chitinous  membranes  connect  the  rings,  and 
make  the  movement  and  expansion  of  the  abdomen  possible. 


FIG.  133.  Sting  of  the  worker  bee.  b,  barbs  on  darts  ;  i,  k,  I,  levers  to  move 
darts;  n,  nerves;  p,  sting-feeler;  pg.,  poison  gland;  ps.,  poison  sac; 
sh.,  sheath;  tfh,  fifth  abdominal  ganglion.  (From  Packard  after 
Cheshire.) 


THE  HONEYBEE  AND   BEES  IN   GENERAL  239 

Each  of  the  last  four  visible  sternal  plates  of  the  worker  bears  a 
pair  of  wax  glands.  At  the  end  of  the  abdomen  of  the  worker  and 
queen  is  the  sting,  and  the  slitlike  openings  of  the  sexual  organs 
and  anus.  There  is  no  sting  in  the  drone,  but  a  copulatory  organ 
is  present. 

THE  STING. — The  sting  is  a  very  complicated  structure  (Fig. 
133).  Before  the  bee  stings,  a  suitable  place  is  selected  with  the 
help  of  the  sting  feelers  (p) ;  then  the  two  barbed  darts  (b)  are 
thrust  forward.  The  sheath  (sh.)  serves  to  guide  the  darts,  to 
open  up  the  wound,  and  to  aid  in  conducting  the  poison.  The 
poison  is  secreted  in  a  pair  of  glands  (pg.)t  one  acid,  the  other  alka- 
line, and  is  stored  in  a  reservoir  (p.s.).  Generally  the  sting,  poison 
glands,  and  part  of  the  intestine  are  pulled  out  when  a  bee  stings, 
so  that  death  ensues  after  several  hours,  but  if  only  the  sting  is 
lost,  the  bee  is  not  fatally  injured.  The  queen  seldom  uses  her 
sting  except  in  combat  with  other  queens. 

The  Anatomy  and  Physiology  of  the  Internal  Organs.  —  GEN- 
ERAL ANATOMY. —  Before  considering  in  detail  the  systems  of 
internal  organs  and  their  functions,  it  may  be  well  to  obtain  a 
general  view  of  their  morphology,  and  point  out  their  resem- 
blances to,  and  differences  from,  those  of  the  crayfish.  Both  the 
bee  and  the  crayfish  have  a  well-developed  muscular  system,  a 
digestive  system  composed  in  the  main  of  similar  parts,  a  dorsal 
blood  vessel  and  a  number  of  sinuses,  a  brain  dorsally  situated 
in  the  head,  and  a  median  ventral  nerve  cord.  The  chief  differ- 
ences are  in  the  excretory  and  respiratory  systems."  No  green 
glands  nor  gills  are  present  in  the  honeybee,  but  in  their  stead 
are  Malpighian  tubules  and  tracheae.  As  in  the  crayfish,  the 
body  cavity  is  not  a  true  ccelom,  the  ccelom  being  restricted  to  the 
cavities  of  the  reproductive  organs. 

THE  MUSCULAR  SYSTEM.  — The  body  of  the  bee  contains  an 
enormous  number  of  muscles;  the  largest  of  these  are  located  in 
the  thorax,  and  are  used  to  move  the  wings  and  legs.  Other  large 
muscles  are  connected  with  the  jaws.  Usually  the  muscles  are 
attached  directly  to  the  inner  surface  of  the  exoskeleton,  often 


240  AN  INTRODUCTION  TO  ZOOLOGY 

by  means  of  hard  tendons.  Muscular  action  is  either  voluntary 
or  involuntary;  for  example,  the  jaws  and  wings  are  moved  by 
voluntary,  many  internal  organs  by  involuntary,  muscles.  The 
strength  of  the  muscles  of  the  bee  is  much  greater  than  that  of 
the  muscles  of  man,  compared  with  their  weight.  The  explana- 
tion of  this  is  quite  simple,  since  the  weight  of  a  muscle  increases 
as  the  cube  of  its  diameter,  while  its  strength  increases  only 
as  the  square.  A  large  animal  can  pull,  therefore,  comparatively 
less  than  a  smaller  one. 

THE  DIGESTIVE  SYSTEM  (Fig.  134). — The  digestive  canal  is 
made  up  of  the  following  structures  named  in  order,  beginning  at 
the  anterior  end:  the  mouth,  oesophagus  or  gullet,  honey  sac  or 
honey  stomach,  true  stomach,  small  intestine  or  ileum,  and  large 
intestine  or  colon.  It  opens  anteriorly  by  the  mouth  and  pos- 
teriorly by  the  anus.  The  (esophagus  (ess.)  is  a  narrow  tube 
which  passes  through  the  thorax;  its  posterior  end  is  enlarged 
into  the  honey  sac  (hs.)  situated  near  the  anterior  end  of  the  abdo- 
men. At  the  posterior  end  of  the  honey  sac  is  the  stomach- 
mouth  (p);  this  structure  extends  slightly  forward  into  the 
honey  sac.  It  has  four  triangular  lips,  which  may  be  opened  or 
closed  by  two  sets  of  muscles,  longitudinal  and  circular.  Near 
the  top  of  the  lips  are  a  number  of  bristles  which  project  back- 
ward. If  the  alimentary  canal  of  a  freshly  killed  bee  is  placed  in 
a  f  to  £  per  cent  salt  solution,  the  lips  will  open  and  close 
rapidly  for  about  half  an  hour. 

The  true  stomach  (c.s.)  is  a  cylindrical  sac;  its  walls  contain  a 
number  of  circular  muscles,  and  a  layer  of  longitudinal  muscles. 
The  digestive  juices  secreted  by  its  walls  change  the  food  into 
chyme.  Part  of  this  chyme  is  absorbed,  the  rest  is  forced  by 
muscular  contractions  into  the  small  intestine.  Undigested  food 
is  dissolved  in  the  small  intestine,  or  ileum  (si.),  and  the  digested 
food  or  chyle  is  absorbed  by  its  walls.  At  its  posterior  end  the 
small  intestine  gradually  merges  into  an  enlargement,  the  colon  (I). 
This  part  of  the  alimentary  canal  receives  all  the  undigested 
matter,  and  discharges  it  to  the  outside  through  the  anus.  The 


FIG.  134.  Internal  organs  of  honeybee,  bt.,  Malpighian  tubules ;  c.s.,  true 
stomach ;  d.v.,  dorsal  vessel ;  e,  eye ;  g,  ganglia  of  nerve-chain ;  7/5., 
honey  sac  ;  //.,  rectum  ;  //>.,  labial  palpus  ;  mesa.  /.,  mesothorax  ;  meta.  /., 
metathorax;  mx.,  maxilla;  n,  nerves;  No.  i,  No.  2,  No.  3,  salivary 
glands;  ces.,  oesophagus;  p,  stomach  mouth;  pro.  /.,  pro  thorax ;  si., 
small  intestine  (ileum)  ;  v,  ventricles  of  dorsal  vessel.  (From  Packard 
after  Cheshire.) 


242  AN   INTRODUCTION  TO  ZOOLOGY 

faces  are  never  voided  within  the  hive  if  the  bees  are  kept  under 
proper  conditions. 

THE  SALIVARY  GLANDS.  —  There  are  two  pairs  of  salivary 
glands,  one  within  the  head,  the  other  within  the  thorax.  Those 
in  the  head  lie  against  the  posterior  walls  of  the  cranium.  The 
other  pair  lie  in  the  ventral  part  of  the  anterior  half  of  the  thorax. 
Both  pairs  of  glands  produce  weakly  alkaline  secretions  which  are 
poured  out  upon  the  labium  where  they  act  upon  the  food  taken 
through  the  proboscis  (Snodgrass,  1910). 

THE  EXCRETORY  ORGANS.  — The  Malpighian  or  urinary  tubules 
(Fig.  134,  bl.)  are  a  number  of  long,  fine,  hairlike  structures  which 
open  into  the  anterior  end  of  the  intestine.  They  were  discovered 
by  and  named  after  the  great  Italian  anatomist  Malpighi.  Ex- 
cretions are  taken  from  the  blood  and  the  fat  body  in  the  form 
of  urates;  they  pass  into  the  intestine,  and  thence  out  of  the 
body. 

THE  VASCULAR  SYSTEM. — The  blood  of  the  honeybee  is  similar 
to  that  of  the  crayfish.  It  is  a  colorless  plasma  containing  ame- 
boid corpuscles.  It  differs  in  one  important  point  from  the  blood 
of  most  other  animals  —  it  does  not  carry  very  much,  if  any, 
oxygen.  The  distribution  of  oxygen  is  accomplished  by  the 
respiratory  system,  as  we  shall  see  later. 

An  even  less  complete  system  of  blood  -vessels  is  present  than 
in  the  crayfish.  The  principal  organ  of  circulation  is  the  dorsal 
vessel,  or  heart  (Fig.  134,  d.v.).  This  is  a  tube  lying  in  the  median 
line  just  beneath  the  dorsal  surface;  it  is  closed  near  the  posterior 
end  of  the  abdomen,  but  opens  in  the  head.  The  walls  of  the 
heart  are  muscular,  and  contract  at  intervals.  Blood  enters 
through  five  pairs  of  ostia,  one  pair  to  each  of  the  five  compart- 
ments or  -ventricles  (v)  into  which  the  heart  is  divided.  Valves 
prevent  the  flowing  back  of  the  blood,  so  that  each  contraction 
sends  a  stream  toward  the  head.  From  here  it  passes  through  the 
spaces  among  the  tissues,  finally  reaching  the  ventral  parts  of  the 
body.  Beneath  the  heart  is  a  horizontal  diaphragm  of  muscle, 
which  forms  a  pericardial  sinus.  By  contracting,  this  diaphragm 


HtTraSc 


VHSp 


FIG.  135. 


THE  HONEYBEE  AND  BEES  IN  GENERAL 


243 


forces  the  olood  from  the  ventral  part  of  the  body  into  the  sinus, 
and  thence  through  the  ostia  into  the  heart. 

THE  RESPIRATORY  SYSTEM  (Fig.  135). — The  honeybee  breathes 
through  openings,  called  spiracles  (sp.),  situated  in  the  sides  of  cer- 
tain thoracic  and  abdominal  segments.  No  air  enters  through 
apertures  in  the  head,  as  in 
vertebrates.  There  are  two 
pairs  of  spiracles  in  the  thorax, 
one  in  the  sides  of  the  pro- 
thorax,  the  other  in  the  meta- 
thorax.  Five  pairs  are  present 
in  the  abdomen.  The  spiracles 
open  into  tubes,  called  trachea. 
These  unite  with  longitudinal 
tracheae  which  extend  along  the 
sides  of  the  body.  Other  tra- 
cheae arise  from  these  longitu- 
dinal trunks  and  distribute  their 
branches  to  all  parts  of  the 
body.  The  tracheae  (Fig.  136) 
are  tubes  composed  of  a  single 
layer  of  cells  (a)  and  lined  with 
a  thin  chitinous  wall.  This  wall 
is  thickened  at  regular  inter- 
vals, forming  a  spiral  thread, 
which  serves  to  keep  the  tra- 
cheae open.  Certain  tracheae 
are  dilated  into  air  sacs  (Fig. 
135),  the  largest  of  which  are  situated  in  the  anterior  part  of  the 

FIG-  135-  Respiratory  system  of  worker  bee  as  seen  from  above,  one  ante- 
rior pair  of  abdominal  sacs  removed  and  transverse  ventral  commissures 
of  abdomen  not  shown.  /  Sp.,  Ill  Sp.,  VII  Sp.,  spiracles;  Ht.  Tra.  Sc.y 
Tra.Sc.,  i,  2,  4,  7,  8,  10,  tracheal  sacs;  Tra.,  tracheae.  (From  Snod- 
grass,  Technical  Series  18,  Bureau  of  Entomology,  United  States  De- 
partment of  Agriculture.) 


FIG.  136.  Portion  of  a  trachea,  a, 
cellular  wall.  (From  Packard 
after  Ley  dig.) 


244 


AN  INTRODUCTION  TO  ZOOLOGY 


abdomen.  These  are  supposed  to  be  of  service  to  the  bee  during 
flight,  since  their  size  can  be  increased  at  will,  and  the  specific 
gravity  of  the  insect,  therefore,  decreased.  Air  is  drawn  into 
and  expelled  from  the  tracheae  by  alternate  expansions  and  con- 
tractions of  the  abdomen.  Each  spiracle  contains  a  valve,  which 
may  be  closed  or  opened.  Dust  is  prevented  from  entering  by 
hairs  which  surround  the  opening. 

The  rate  of  respiration  depends  upon  the  activity  of  the  indi- 
vidual. Normally  there  are  about  forty  inspirations  per  minute, 
but  in  fatigued  bees  the  number  reaches  as  high  as  one  hundred 
and  sixty  per  minute.  The  air  brought  into  the  bee's  body  is 
carried  by  the  tracheae  directly  to  the  tissues,  no  circulatory  sys- 
tem being  necessary  for  the  distribution  of  oxygen. 

THE  NERVOUS  SYSTEM  (Fig.  137,  Op.  L.-j  Gng.). — The  nervous 
system  of  the  bee  consists  of  a  large  chain  of  paired  ganglia  and 
two  groups  of  smaller  ganglia,  called  the  stomatogastric,  and  the 
sympathetic,  respectively.  The  large  ganglionic  chain  is  by  far 
the  most  important.  It  is  formed  of  seven  masses  of  nervous 
tissue,  two  in  the  head,  two  in  the  thorax,  and  live  in  the  abdo- 
men. Each  mass  is  composed  of  two  ganglia  lying  side  by  side, 
and  connected  with  the  mass  in  front  and  behind  by  two  nerve 
cords.  All,  except  the  foremost  ganglionic  mass,  are  situated 
near  the  center  of  the  ventral  body  wall.  The  ganglia  at  the 
extreme  anterior  end  occupy  a  cavity  in  the  dorsal  part  of  the 
head;  they  are  known  as  the  brain,  or  supraoesophageal  ganglia. 
Nerves  connect  the  brain  with  the  compound  eyes  (Fig.  137,  Op.  L.), 
the  ocelli,  the  antennae,  and  the  labrum.  Beneath  the  oesopha- 
gus in  the  head  lies  the  subcesophageal  ganglion  which  innervates 
the  mandibles,  labium,  and  other  mouth  parts.  The  anterior 

FIG.  137.  Longitudinal,  median,  vertical  section  of  entire  body  of  worker 
bee,  showing  nervous  system  (Op.  L.-?Gng.),  tracheal  system  (Tra.  Sc. 
i-io),  dorsal  and  ventral  diaphragms  of  abdomen  (D.  Dph.  and  V.  Dph.}, 
and  dorsal  vessel  consisting  of  heart  (Ht.}  and  aorta  (Ao.}.  (From 
Snodgrass,  Technical  Series  18,  Bureau  of  Entomology,  United  States 
Department  of  Agriculture.) 


o 


THE  HONEYBEE  AND  BEES  IN  GENERAL 


245 


ganglion  in  the  thorax  sends  nerves  into  the  front  pair  of  legs. 
The  posterior  thoracic  ganglion  is  comparatively  large,  consisting 
really  of  several  ganglia  which  have  grown  together.  The  an- 
terior part  of  this  ganglion  supplies  the  fore  wings  and  the  middle 
pair  of  legs;  the  posterior  part  innervates  the  hind  wings  and 
legs.  Various  parts  of  the  abdomen  are  supplied  with  nerves 
from  the  abdominal  ganglia;  the  last  of  these  is  larger  than  the 
others,  because  of  the  important  organs,  the  genital  apparatus 
and  the  sting,  which  are  innervated  by  them. 

The  stomato-gastric  part  of  the  nervous  system  is  made  up  of 
many  small  ganglia  connected  with  the  organs  of  digestion,  cir- 
culation, and  respiration. 

Each  segment  of  the  body  contains  a  triangular  ganglion  from 
which  fine  nerve  fibers  pass  to  all  parts  of  the  body.  This  is  the 
so-called  sympathetic  nervous  system. 

THE  SENSORY  ORGANS. — THE  EYES  AND  VISION. — p:ach 
compound  eye  is  made  up  of  a  great  number  of  long,  slender 
structures  called  ommatidia.  There  may  be  as  many  as  five 
thousand  of  these  in  a  single  eye.  The  ommatidia  are  all  alike 
in  structure.  They  may  be  recognized  externally  as  minute 
hexagonal  areas  or  facets,  among  which  arise  long  protective  hairs 
which  are  unbranched.  Passing  from  without  in,  the  omma- 
tidium  is  found  to  contain  the  following  parts:  the  cornea;  the 
crystalline  cone  composed  of  four  modified  cells  surrounded  by 
two  cells  containing  coloring  matter  ;  and  the  rhabdome,  a  deli- 
cate transparent  rod  surrounded  by  eight  slender  retinular 
cells,  and  about  twelve  pigment  cells,  which  extend  to  a  basal 
membrane.  In  all  there  are  twenty-eight  parts  to  each 
ommatidium  (216).  The  pigment  cells  prevent  the  reflection 
of  light  within  the  ommatidium  and  between  neighboring 
ommatidia. 

The  ocelli,  though  commonly  known  as  simple  eyes,  are  almost 
as  complex  as  the  compound  structures  just  described.  Each 
ocellus  consists  of  an  extremely  convex  cornea,  and  a  large  bi- 
convex crystalline  cone,  behind  which  are  a  great  number  of  rods 


246  AN  INTRODUCTION  TO  ZOOLOGY 

resembling  somewhat  the  ommatidia.     An  optic  nerve  from  the 
brain  passes  to  each  ocellus. 

VISION.  —  A  number  of  interesting  biological  problems  are 
directly  concerned  with  the  vision  of  the  honeybee  and  other 
insects;  among  these  may  be  mentioned  the  origin  of  flowers  and 
cross-pollination,  the  method  of  rinding  the  way  back  to  the 
hive,  and  the  finding  of  the  queen  by  the  drone  during  the  nuptial 


FIG.  138.  Longitudinal  section  through  part  of  an  antenna  of  the  honey- 
bee, c,  conoid  hairs;  /,  tactile  hairs;  ho.,  auditory  pits;  n,  nerves; 
s,  smell  hollows.  (From  Cheshire.) 

flight.  These  questions  cannot  be  definitely  answered  because 
the  exact  character  of  the  image  produced  by  the  eyes  is  not 
known.  A  modification  of  the  "  mosaic  "  theory,  proposed  by 
Miiller  in  1826  and  described  for  the  compound  eye  of  the  cray- 
fish on  page  207,  is  still  held  by  many  investigators.  Movements 
are  made  known  to  the  bee  very  quickly,  according  to  this  theory, 
and  the  form  of  objects  while  the  insect  is  moving  are  likewise 
instantly  perceived,  since  the  various  facets  would  be  affected 
in  succession. 

There  is  experimental  evidence  that  the  ocelli  enable  the 
insect  to  distinguish  light  from  darkness.  If  images  are  formed 
by  them,  they  must  be  of  objects  at  a  definite  range,  since  the 
lens  is  incapable  of  accommodation.  Furthermore,  the  great 
convexity  of  the  lens  makes  it  probable  that  form  can  be  perceived 


THE  HONEYBEE  AND  BEES  IN  GENERAL 


247 


by  ocelli  at  only  very  short  distances,  i.e.  the  bee  is  nearsighted 
when  only  the  ocelli  are  used. 

SMELL.  —  There  seems  to  be  no  doubt  but  that  the  principal 
organs  of  smell  in 
bees  are  situated 
on  the  antennae. 
Other  parts  of  the 
body  also  contain 
organs  which  have 
been  considered 
by  some  to  be 
concerned  with 
the  perception  of 
odors.  The  struc- 


tures supposed  to 


FIG.  139.   Taste  pits  (/?)  on  the  epipharynx  of  the 
honeybee.     (From  Packard  after  Wolff.) 


carry  on  the  func- 
tion of  smell  are  shown  in  longitudinal  section  in  Figure  138  at  s. 
They  are  slight  hollows  covered  by  a  thin 
layer  of  chitin  and  provided  at  their  bases 
with  a  cell  supplied  with  nerve  fibers. 
The  number  of  smell  hollows  varies  for 
different  members  of  the  bee  colony;  the 
queen  possesses  about  1600  on  each  an- 
tenna, the  worker  2400,  and  the  drone 
37,800.  This  enormous  number  on  the 
drone  probably  aids  him  in  finding  the 
queen  during  the  nuptial  flight  (198). 

TASTE.  —  Certain   structures   situated 
near  the  mouth  of  the  honeybee  have  been 
described  as  taste  organs.  The  epipharynx 
(Fig.  139)  contains  a  number  of  sensory 
cavities  which  are  considered   gustatory 
by  some  investigators.     Taste  setae   are 
also  present  near  the  end  of  the  tongue  (Fig.  140,  Gs.). 
HEARING.  —  The  antennae  bear  a  large  number  of  pits  (Fig. 


'IG.  140.  Taste  hairs  on 
tongue  of  bee.  Gs., 
taste  hairs;  L,  bou- 
ton.  (From  Pack- 
ard after  Will.) 


248  AN  INTRODUCTION  TO  ZOOLOGY 

138,  ho.)  supposed  to  be  the  end  organs  of  hearing.  Each  pit 
has  a  cone  at  its  base  connected  with  a  sensory  cell.  It  is  very 
doubtful,  however,  whether  bees  have  any  sense  of  hearing, 
since  the  exact  functions  of  antennal  organs  is  not  known  in 
any  case. 

The  sounds  made  by  bees  may  be  entirely  incidental  to 
other  activities.  Sounds  result  from  the  vibration  of  the  wings, 
the  vibration  of  the  abdominal  segments,  and  the  activity  of 
the  spiracular  vocal  apparatus.  The  size  of  the  wings  and 
physiological  condition  of  the  bee  determine  the  rate  of  vibra- 
tion, and  consequently  the  pitch.  When  in  full  flight,  the  440 
vibrations  per  second  give  a  in  the  treble  clef,  but,  if  fatigued, 
only  330  vibrations  per  second  may  be  produced,  giving  e.  The 
so-called  vocal  apparatus  lies  within  the  spiracular  openings 
of  the  respiratory  system.  It  consists  of  a  vocal  membrane, 
a  sounding  box,  and  a  mechanism  for  regulating  the  size  of  the 
opening.  Air  passing  to  the  outside  vibrates  the  membrane 
producing  a  humming  sound. 

TOUCH.  —  Bees  possess  a  tactile  sense,  the  end  organs  of  which 
are  hairlike  structures  on  various  parts  of  the  body,  but  especially 
on  the  antennae.  At  least  two  kinds  of  tactile  organs  occur  on 
the  antennas.  One  of  these  consists  of  a  small  hair  (Fig.  138,  /) 
which  projects  through  a  minute  opening  in  the  ch'.tin,  and  is 
connected  with  a  nerve  cell  (n)  within.  The  other  touch 
organs  are  termed  conoid  hairs  (Fig.  138,  c');  they  are  larger, 
and  have  a  central  cavity  containing  a  nerve  fiber.  More 
tactile  hairs  are  present  near  the  ends  of  the  antennae  than 
elsewhere. 

The  Reproductive  System.  —  MALE  REPRODUCTIVE  ORGANS 
(Fig.  141).  —  In  the  abdomen  of  the  drone  are  two  testes  (Tes.), 
each  consisting  of  about  three  hundred  spermatic  tubes  in  which 
the  spermatozoa  are  formed;  they  are  connected  by  a  pair  of  fine 
tubes,  the  vasa  defer entia  (V.  Def.),  with  the  seminal  vesicles (Ves.). 
The  seminal  vesicles  open  into  a  pair  of  large  mucous  glands 
?.),  which  unite  at  the  point  where  the  ejaculatory  duct  (Ej.D.) 


o,m 


OvD 

/ 
/ 


Spm 


SpmC 


AG1E 


BCp 


AG1 


FIG.  142. 


THE  HONEYBEE  AND   BEES  IN   GENERAL  249 


Pen  AcGl 


FIG.  141.  Reproductive  organs  of  drone  bee,  dorsal  view,  natural  position. 
Ac.  Gl.,  accessory  gland;  B.,  bulb  of  penis;  Ej.  D.,  ejaculatory  duct; 
Pen.,  penis;  Tes.,  testis;  V.  Def.,  vas  deferens;  Ves.,  seminal  vesicle; 
//.,  uu.,  yy.,  zz.,  parts  of  the  penis.  (From  Snodgrass,  Technical 
Series  18,  Bureau  of  Entomology,  United  States  Department  of  Agri- 
culture.) 

begins.      At  the  posterior  end,  the  ejaculatory  duct  enters  the 
copulatory  organ  (Pen.}. 

FEMALE  REPRODUCTIVE  ORGANS  (Fig.  142).  — As  stated  on  page 
233,  the  queen  lays  all  of  the  eggs,  the  workers  being  sexually 

FIG.  142.  Reproductive  organs,  sting,  and  poison  gland  of  queen.  A.  Gl., 
acid  gland ;  A.  Gl.  D.,  duct  of  acid  gland  ;  B.  GL,  alkaline  gland  ;  Ov., 
ovary  ;  ov.,  ovarian  tubule  ;  Ov.  D.,  oviduct ;  Psn.  Sc.,  poison  sac  ;  Spm., 
spermatheca ;  Sin.,  sting  ;  Stn.  Pip.,  sting  feeler  ;  Vag.,  vagina.  (From 
.Snodgrass,  Technical  Series  18,  Bureau  of  Entomology,  United  States 
Department  of  Agriculture.) 


250  AN   INTRODUCTION  TO   ZOOLOGY 

undeveloped  females.  The  latter,  however,  contain  vestigial 
reproductive  organs  which  may  even,  under  certain  conditions, 
become  capable  of  producing  eggs.  The  abdomen  of  the 
queen  is  almost  completely  filled  by  the  two  ovaries  (Fig.  142,  Ov.). 
Each  ovary  is  made  up  of  a  great  number  of  ovarian  tubules  (ov.) 
containing  eggs  of  various  sizes,  the  largest  at  the  posterior  end. 
Eggs  pass  from  these  tubules  into  the  oviducts  (Ou.D.),  thence 
into  the  vagina  (Vag.),  and  out  of  the  body  by  way  of  the  genital 
aperture.  Opening  into  the  vagina  is  a  spherical  sac,  the  sper- 
maiheca  (Spm.),  rilled  with  spermatozoa  received  from  the  male 
during  copulation. 

SPEKMATOGENESIS.  —  The  maturation  of  the  male  cells  of  the 
honeybee  differs  markedly  from  the  usual  type  (p.  103),  and 
from  that  of  the  animals  thus  far  described.  The  primordial 
germ  cells  grow  into  spermatogonia  as  usual.  The  first  sper- 
matocyte  division,  however,  which  ordinarily  results  in  two  sec- 
ondary spermatocytes  of  equal  size,  is  a  sort  of  budding  process. 
A  small  portion  of  the  cytoplasm  is  pinched  off  and  disintegrates. 
The  cell  remaining,  the  secondary  spermatocyte,  retains  all  of 
the  chromatin  originally  contained  in  the  spermatogonium.  The 
secondary  spermatocyte  now  divides,  producing  one  small  cell 
with  half  of  the  chromatin,  but  very  little  cytoplasm,  and  one 
large  cell.  The  small  cell  begins  to  develop  into  a  spermatozoon, 
but  probably  degenerates.  The  larger  cell,  which  may  now  be 
called  a  spermatid,  metamorphoses  into  a  single  functional 
spermatozoon  (209,  210). 

OOGENESIS.  —  It  is  now  pretty  well  established  that  the  eggs 
which  produce  drones  are  not  fertilized,  while  those  that  produce 
the  workers  and  queens  are.  The  ripening  of  the  latter  is  similar 
to  this  process  in  other  animals,  but  the  maturation  of  the  unfer- 
tilized "  drone  egg  "  is  unique.  A  full  account  of  this  process 
has  been  published  by  Petrunkewitch  (215). 

COPULATION.  —  The  spermatozoa  are  transferred  from  the 
drone  to  the  queen  while  the  latter  is  taking  her  nuptial  flight. 
Usually  from  five  to  eight  days  after  the  queen  emerges  from  her 


THE  HONEYBEE  AND  BEES  IN  GENERAL 


251 


cell,  she  ventures  out  of  the  hive,  first  crawling  about  near  the 
entrance,  then  taking  short  flights,  and  finally  her  wedding  trip 
of  from  three  to  thirty  minutes.  She  is  followed  by  the  drones, 
one  of  which  copulates  with  her.  The  result  of  copulation  is  the 
filling  of  the  spermatheca  of  the  queen  with  spermatozoa  (Fig. 
143).  She  usually  copulates  only  once,  the  sperm  obtained  at 
that  time  being  sufficient  to  fertilize  thousands  of  eggs.  After 
her  nuptial  flight  the  queen  never 
leaves  the  hive  except  with  a  swarm. 

FERTILIZATION.  — The  eggs  are  fer- 
tilized just  before  deposition.  How 
this  is  accomplished  is  not  definitely 
known.  The  queen  seems  to  be  able 
to  lay  fertilized  or  unfertilized  eggs 
according  to  the  size  of  the  cell  in 
which  the  individual  is  to  develop, 
but  it  has  been  proven  that  the  size 
of  the  cell  does  not  automatically 
determine  this.  Fertilized  eggs  de- 
velop into  queens  and  workers, 
whereas  the  unfertilized  eggs  which 
develop,  become  drones.  The  method 
of  control  of  fertilization  is  still  a 
mystery. 

EGG  LAYING.  —  The  eggs  are  bluish  white,  and  oblong  in  shape; 
they  are  deposited  by  the  queen  at  the  base  of  the  cells  and  fas- 
tened in  a  position  parallel  to  the  sides  of  the  cells  by  a  glutinous 
secretion.  The  fertilized  eggs  are  laid  either  in  small  worker 
cells,  or  in  large  irregular  queen  cells.  Unfertilized  eggs  are 
usually  laid  in  drone  cells. 

Embryology.  — The  fertilized  egg  is  made  up  of  a  large  central 
mass  of  yolk  spheres,  among  which  are  traces  of  cytoplasm,  and 
a  peripheral  layer  of  cytoplasm.  A  single  nucleus  is  present; 
this  is  the  nucleus  of  the  egg  and  that  of  the  spermatozoon  com- 
bined. A  chitinous  shell,  the  chorion,  surrounds  the  egg;  this  is 


FIG.  143.  Spermatheca  of 
queen  honeybee.  a, 
space  filled  with  fluid  ; 

b,  mass  of  spermatozoa  ; 

c,  duct ;  d,  active  sper- 
matozoa.   (From  Pack- 
ard after  Cheshire.) 


252  AN  INTRODUCTION  TO  ZOOLOGY 

lined  by  a  delicate  vitettine  membrane.  As  in  the  crayfish,  cleav- 
age proceeds  without  the  intervention  of  cell  walls.  Most  of  the 
cleavage  nuclei  migrate  to  the  periphery,  where  they  form  a  single 
layer  of  cells,  the  blastoderm;  the  rest  remain  in  the  yolk,  which  it 
is  their  duty  to  change  into  protoplasm  as  development  continues. 
The  blastoderm  soon  becomes  thicker  on  one  side  of  the  egg, 
forming  the  germ  band  or  primitive  streak.  This  later  becomes 


FlG.  144.  Embryo  of  honeybee  within  eggshell,  ab.,  hind-intestine;  c, 
cesophageal  connectives ;  ch.,  chorion  or  eggshell ;  fb.,  fore-intestine ; 
ga.,  ganglia ;  mb.,  mid-intestine ;  s.  ga.,  brain.  (From  Packard  after 
Cheshire.) 

the  ventral  side  of  the  young  bee.  Next  a  median  groove  appears 
in  the  germ  band  and  two  layers  of  cells  arise  in  this  region;  the 
outer  layer  is  the  ectoderm,  the  inner,  the  entomesoderm.  The 
latter,  as  its  name  implies,  gives  rise  to  both  the  entoderm  and 
mesoderm.  The  germ  band  now  grows  around  the  egg  until  it 
covers  the  entire  surface.  The  antennae  and  four  pairs  of  append- 
ages appear  near  the  anterior  end  of  the  embryo.  One  pair  of 
the  latter  disappear;  the  others  become  the  mouth  parts.  Three 
pairs  of  appendages  also  develop  on  the  thorax,  but  disappear 
before  the  embryo  hatches  (200).  Some  of  the  organs  of  the 
embryo  are  shown  in  Figure  144.  The  ganglia  of  the  brain 
(s.  ga.)  and  ventral  nerve  cord  (ga.)  are  quite  distinct  at  this  time, 
and  the  three  principal  parts  of  the  alimentary  canal,  the  fore- 
intestine  (fb.),  mid-intestine  (mb.),  and  hind-intestine  (ab.)  occupy 
the  longitudinal  axis  of  the  body. 

Metamorphosis. — The  life  history  of  an  individual  may  be 
divided  into  four  periods,  egg,  larva,  pupa,  and  adult  or  imago. 


ffff 


FIG.  145.  Larvae  and  pupa  of  honeybee  in  their  cells.  SI.,  larva  spinning 
cocoon  ;  N,  pupa  ;  PL.,  young  larva  ;  an.,  antenna  ;  ce.,  eye  ;  co.,  cocoon; 
m,  mandible  ;  sp.,  spiracles  ;  t,  tongue  ;  w,  wing.  (From  Packard  after 
Cheshire.) 


FIG.  146.  Internal  organs  of  larval  honeybee,  an.,  anus;  bin.,  nerve  chain  ; 
cd.,  true  stomach ;  ed.,  hind-intestine ;  g,  brain ;  st.,  spiracles ;  a?, 
oesophagus  ;  sd.,  spinning  gland  ;  vm.,  Malpighian  tubule.  (From  Pack- 
ard after  Leuckart.) 


THE  HONEYBEE  AND  BEES  IN  GENERAL 


253 


The  length  of  time  of  each  state  is  shown  in  Table  XI  (191, 
p.  28). 

TABLE  XI 

THE  TIME  OCCUPIED   IN  THE  DEVELOPMENT    OF    QUEEN,  WORKER,  AND   DRONE 


EGG 

LARVA 

PUPA 

TOTAL  FROM  DEPOSITION 

OF  EGG  TO  ADULT 

Days 

Days 

Days 

Days 

Queen       .     . 

3 

51A 

7 

isH 

Worker    .     . 

3 

5 

i3 

21 

Drone      .     . 

3 

6 

i5 

24 

After  three  days  the  larva  emerges  from  the  egg,  and  lies  at 
the  base  of  the  cell  (Fig.  145,  FL.)  floating  in  the  food  prepared  by 
the  workers  and  known  as  chyle  or  "  bee  milk."  Chyle  is  com- 
posed of  digested  honey  and  pollen,  probably  mixed  with  a 
glandular  secretion,  and  is  given  to  all  of  the  larvae  by  the  nurse 
bees  during  the  first  three  days.  Then  the  larvae  that  will  be- 
come workers  are  given  honey  and  digested  pollen  in  gradually 
increasing  amounts;  the  drone  larvae,  after  the  fourth  day,  also 
receive  honey  and  undigested  pollen;  but  the  queen  larvae  are  fed 
lavishly  on  the  rich  albuminous  bee  milk,  the  "  royal  jelly," 
until  they  change  to  pupae. 

GROWTH  during  the  larval  period  is  accompanied  by  several 
moults  of  the  chitinous  larval  envelope.  At  the  end  of  the  larval 
period  the  cells  containing  the  young  brood  are  covered  over 
with  wax,  feeding  ceases,  and  the  larvae  proceed  to  spin  a  cocoon 
of  silk  from  their  spinning  glands  (Fig.  145,  SL.).  These  spinning 
glands  (Fig.  146,  sd.)  become  the  salivary  glands  (systems  I  and 
II)  of  the  adult.  The  simple  structure  of  the  larva  is  shown  in 
Figure  146.  The  alimentary  canal  consists  of  an  oesophagus 
(03),  a  chyle  stomach  (cd.),  a  hind-intestine  (ed.),  and  two  sets  of 
appendages,  the  spinning  glands  (sd.),  and  the  Malpighian  tubules 
(vm.).  Almost  every  segment  contains  a  pair  of  spiracles  (st.), 
and  a  ganglion  of  the  central  nervous  system  (bm.). 


254  AN  INTRODUCTION  TO  ZOOLOGY 

It  takes  the  worker  thirty-six  hours  to  spin  its  cocoon,  then  it 
slowly  changes  into  a  pupa,  or  chrysalis  (Fig.  145,  N).  Practi- 
cally the  entire  body  is  made  over  at  this  time;  the  three  regions, 
head,  thorax,  and  abdomen,  become  distinct;  externally  the  wings 
(w),  legs,  mouth  parts  (t,  m),  sting,  antennae  (an.),  and  eyes  are 
visible;  and  the  internal  changes  are  even  more  striking,  the 
larval  organs  developing  into  those  of  the  adult,  and  new  organs 
appearing.  After  a  period  of  rest  the  pupa  casts  off  its  exoskele- 
ton,  and  emerges  as  an  adult. 

The  Activities  of  the  Workers.  —  The  functions  of  the  queen 
and  drone  are  few  as  compared  with  those  of  the  worker.  The 
queen  lays  eggs,  and  the  drone  fertilizes  the  queen.  But  the 
workers  have  a  large  number  of  varied  activities,  such  as  the  build- 
ing of  honeycomb,  the  collection  of  propolis  or  "  bee  glue " 
with  which  the  inside  of  the  hive  is  varnished,  the  gathering  of 
pollen  or  "  bee  bread  "  and  its  preparation  as  food,  the  feeding 
of  the  queen  and  young  bees,  the  carrying  of  water,  the  collec- 
tion of  flower  nectar  and  its  manufacture  into  honey,  and  the 
cleaning,  warming,  ventilating,  and  guarding  of  the  hive.  Work- 
ers also  accomplish  the  pollination  of  flowers,  raise  new  queens 
when  necessary,  and  increase  the  number  of  colonies  by  swarming. 

THE  BUILDING  OF  HONEYCOMB. — Wax  is  produced  by  pairs 
of  wax  glands  on  the  sterna  of  the  last  four  visible  abdominal 
segments  of  the  worker.  Honey  and  pollen  are  consumed  in  the 
process,  and  a  temperature  of  from  97°  to  98°  F.  is  maintained. 
When  comb  is  to  be  built,  the  bees  gorge  themselves  with  honey, 
and  hang  in  dense  clusters  from  the  top  of  the  hive  for  several 
hours.  Thin  scales  of  wax  are  then  secreted  by  the  wax  glands, 
removed  by  the  wax  pinchers  on  the  metathoracic  legs,  trans- 
ferred to  the  prothoracic  legs,  and  then  to  the  mouth,  where  they 
are  mixed  with  saliva  and  kneaded  by  the  jaws.  If  a  new  comb 
is  to  be  built,  the  wax  is  plastered  to  the  roof,  and  in  some 
mysterious  way  each  bee  puts  its  contribution  almost  exactly 
where  it  is  to  remain.  The  cells,  which  are  gradually  built  up, 
are  hexagonal  in  shape. 


THE  HONEYBEE  AND  BEES  IN  GENERAL 


255 


Cells  differ  in  size  according  to  their  uses.  There  are  six 
kinds  —  worker,  drone,  queen,  transition,  attachment,  and 
honey.  The  cells  in  which  the  workers  are  reared  measure 
one  fifth  of  an  inch  between  the  parallel  sides.  The 
drone  cells  are  larger,  measuring  one  fourth  of  an  inch 


Drone  cells 


Transition  cells 
A 


Worker  cells 


FIG.  147.  Honeycomb  showing  various  kinds  of  cells.  A,  diagram 
showing  comparative  size  of  drone  cells  and  worker  cells.  B,  photo- 
graph of  a  piece  of  honeycomb  showing  circular  cells  and  attachment 
cells.  (From  Root.) 


between  the  parallel  sides.  At  certain  times  some  of  the 
hexagonal  cells  are  torn  down  and  a  large  queen  cell  is  built. 
Between  the  worker  and  drone  cells  is  a  zone  of  irregular  transi- 
tion cells.  The  cells  which  fasten  the  comb  to  the  top  or  sides  of 
the  hive  are  called  attachment  cells.  Honey  is  stored  not  only  in 


256  AN  INTRODUCTION  TO  ZOOLOGY 

honey  cells,  but  also  in  drone,  worker,  and  transition  cells.  Care- 
ful measurements  have  shown  that  the  cells  are  seldom  perfectly 
symmetrical,  although  in  many  cases  they  appear  so  to  our  eyes. 
The  honey  cells  are  built  with  entrances  slightly  above  their  bases, 
so  that  the  honey  stored  in  them  will  not  flow  out  before  it  be- 
comes "  ripe"  (200). 

THE  COLLECTION  OF  PROPOLIS. — "Bee  glue,"  as  propolis  is 
sometimes  called,  is  a  resinous  material  collected  from  buds 
and  crevices  of  trees.  It  is  transported  in  the  pollen  baskets,  and 
is  used,  as  soon  as  collected,  to  paint  the  inside  of  the  hive,  to 
fill  up  cracks,  and  to  strengthen  any  loose  parts. 

GATHERING  POLLEN.  —  Pollen  grains  are  of  inestimable  value 
in  the  bee  household.  They  are  very  small,  of  various  shapes  and 
colors,  and  are  formed  within  a  part  of  the  flower,  known  as  the 
anther.  The  pollen  grains  contain  the  male  element  in  the  fertili- 
zation of  flowers,  and  consequently  are  necessary  for  the  produc- 
tion of  fertile  seed.  To  the  bee,  pollen  is  invaluable  as  a  food, 
and  is  also  used  in  preparing  the  cells  containing  pupae.  We 
have  already  described  the  peculiar  structures  on  the  legs  and 
other  parts  of  the  bee's  body  used  in  collecting  pollen  (p.  236). 
Upon  reaching  the  hive  the  pellets  of  pollen  are  pried  out  of  the 
pollen  basket  by  the  spur  at  the  termination  of  the  tibia  of  the 
middle  leg  (Fig.  131,  D,  s),  and  deposited  outside  the  brood 
clusters  in  whatever  cells  are  available  —  usually  in  worker  cells. 
Pollen  is  the  principal  food  of  the  larvae.  It  is  very  rich  in 
nitrogenous  material,  a  food  element  not  found  in  honey,  and 
without  which  the  young  would  starve.  The  gathering  of  pollen 
by  bees  has  a  great  influence  upon  the  flowers  visited,  as  is  ex- 
plained in  another  place  (p.  262). 

CARRYING  WATER.  —  During  warm  weather  water  is  sucked  up 
into  the  honey  sac  from  dew,  or  brooks  and  pools,  and  carried 
to  the  larvae  in  the  hive.  In  cool  weather  moisture  condenses 
in  the  hive  in  sufficient  quantities,  and,  under  some  conditions, 
to  such  an  extent  as  to  injure  the  inhabitants. 

THE  MANUFACTURE  OF  HONEY. — Bees  do  not  collect  honey  from 


THE  HONEYBEE  AND   BEES  IN  GENERAL  257 

flowers,  but  gather  nectar,  which  is  later  transformed  into  honey. 
The  nectar  is  lapped  up  by  the  tongue  (Fig.  129,  /),  and  trans- 
ferred to  the  honey  sac  (Fig.  134,  hs.),  where  it  is  stored  while  the 
bee  is  in  the  field.  Part  of  the  water  contained  in  the  nectar 
may  be  excreted  before  the  hive  is  reached.  Nectar  is  placed 
in  open  cells  in  the  well- ventilated  hive  until  all  but  18  to  20  per 
cent  of  the  water  contained  in  it  has  evaporated.  When  a  cell 
is  finally  filled  with  "  ripe  "  honey,  it  is  sealed  with  a  cap  of  wax. 

The  flavor  of  honey  depends  upon  the  kind  of  flowers  from 
which  the  nectar  is  collected.  "  Among  the  most  important 
producers  of  the  best  honey  in  the  East  and  North  are  white  clover, 
basswood.  buckwheat,  and  the  fruit  trees  and  small  fruits;  in 
the  middle  states  are  the  tulip  tree,  sorrel  tree,  sweet  clover,  and 
alfalfa;  in  the  South  are  the  mangrove,  cabbage  and  saw 
palmettos,  and  sorrel  tree;  while  in  the  West  are  alfalfa  and  white 
sage.  The  best  and  most  of  the  California  honey  is  from  the 
wild  white  sage  "  (207,  p.  529).  The  amount  of  honey  produced 
in  one  hive  in  a  fair  season  ranges  from  an  average  of  about 
thirty  pounds  of  comb  honey  to  possibly  fifty  pounds  of  ex- 
tracted honey.  This  will  net  the  beekeeper  from  ten  to  fifteen 
cents  per  pound  (218). 

CLEANING  THE  HIVE.  — The  health  of  the  swarm  depends  upon 
the  cleanliness  of  their  domicile,  since  perfect  sanitary  conditions 
are  necessary  where  so  many  individuals  live  in  such  close  quar- 
ters. Dead  bees,  pieces  of  old  comb,  the  excreta  of  the  queen, 
drones,  and  others  that  remain  in  the  hive,  and  any  other  waste 
material  is  immediately  removed. 

VENTILATING  THE  HIVE.  —  Fresh  air  for  the  hive  is  obtained 
by  the  exertions  of  certain  of  the  workers.  Many  bees  near  the 
entrance,  and  at  other  places  in  the  hive,  are  busily  engaged  in 
vibrating  their  wings,  and  creating  a  current  of  air,  which  keeps 
the  hive  fresh,  and  aids  in  ripening  the  nectar.  The  loud  buzzing 
which  accompanies  this  activity  is  often  heard  at  night  after  a 
large  amount  of  nectar  has  been  collected  (191). 

GUARDING  THE  HIVE.  —  The  hive  is  guarded  against  the  intru- 


258  AN  INTRODUCTION  TO  ZOOLOGY 

sions  of  yellow  jackets,  bee  moths,  and  other  bees  by  workers, 
who  wander  back  and  forth  near  the  entrance,  and  examine  every 
creature  that  visits  the  colony.  If  the  swarm  is  strong,  the  guards 
succeed,  with  the  aid  of  the  beekeeper,  in  warding  off  all  honey- 
loving  enemies. 

SWARMING.  —  The  number  of  bees  in  a  hive  increases  very 
rapidly,  since  the  queen  usually  lays  from  950  to  1200  eggs  per 
day.  When  the  colony  is  in  a  prosperous  condition,  and  there 
is  danger  of  overcrowding,  queen  cells  are  built  by  the  workers, 
usually  around  the  fertilized  eggs,  and  new  queens  are  reared. 
Two  queens  do  not  live  amicably  in  one  hive,  and,  if  such  a  con- 
dition arises,  either  there  is  a  battle  between  the  two,  resulting  in 
the  death  of  one  of  them,  or  the  workers  kill  one,  or  else  the  old 
queen  collects  from  two  to  twenty  thousand  workers  about  her 
and  flies  away  with  them  to  found  a  new  colony.  This  is  known 
as  swarming.  The  old  hive  is  not  broken  up,  but  continues  its 
existence  as  before. 

Swarming  occurs  in  May,  June,  or  July,  according  to  latitude, 
and  a  second  swarming  period  may  be  inaugurated  if  weather 
conditions  result  in  a  midsummer  flow  of  honey.  Before  issuing 
from  the  hive,  the  honey  sacs  are  filled  with  honey  to  serve  until 
a  new  home  is  found.  The  swarm,  after  flying  a  short  distance, 
comes  to  rest  upon  the  limb  of  a  tree  or  other  object  where  it 
remains  sometimes  for  several  hours.  A  site  for  the  new  colony 
is  sometimes  chosen  by  scouting  bees  several  days  before  the 
swarm  leaves  the  parent  hive.  These  scouts  may  also  partially 
prepare  the  place  by  cleaning  out  loose  dirt,  bark,  etc.  The 
usual  choice  is  a  hollow  tree,  such  as  the  wild  ancestors  of  the 
honeybee  inhabited,  and  henceforth  is  called  a  "  bee  tree."  One 
of  the  duties  of  the  beekeeper  is  to  hive  the  swarms  before  they 
succeed  in  escaping  to  the  woods.  Swarms  may  also  be  formed 
artificially. 

The  Enemies  of  the  Honeybee.  —  Weak  or  neglected  hives 
may  be  attacked  by  the  Bee  Moth,  Galleria  mellonella,  which 
takes  advantage  of  every  opportunity  to  enter  and  lay  its  eggs. 


THE  HONEYBEE  AND   BEES  IN   GENERAL  259 

The  larvae  feed  principally  on  pollen,  and  the  cocoons  and  cast- 
off  larval  skins  in  the  brood  combs.  They  make  burrows  in  the 
comb  and  line  them  with  silk  as  a  protection  from  the  bees. 

The  bee  louse,  Braula  cceca,  is  parasitic  on  bees  in  Mediter- 
ranean countries,  but  thus  far  has  not  gained  a  foothold  in 
America.  The  bee  lice  may  weaken  the  queen  by  sucking  the 
juices  from  her  body.  Other  insects,  such  as  dragon  flies,  ants, 
and  wasps,  attack  bees,  especially  in  tropical  and  subtropical 
regions.  Spiders  frequently  capture  bees  in  their  webs. 

Birds  are  accused  of  using  honeybees  for  food,  and  one  species, 
the  kingbird,  is  called  the  "  bee  martin,"  because  of  its  supposed 
fondness  for  them.  The  percentage  of  honeybees  eaten  by 
kingbirds  is,  however,  very  small,  and  amply  repaid  by  the  many 
other  insects  they  devour. 

Toads  and  lizards  are  important  enemies  of  the  honeybee,  but 
should  not  be  destroyed  when  captured  near  the  hives,  since  their 
removal  to  a  safe  distance  will  prevent  them  from  devouring  bees 
and  give  them  a  chance  to  be  of  benefit  by  destroying  noxious 
insects.  Mice  prey  upon  pollen,  honey,  and  bees  during  the 
winter.  Hives  also  need  to  be  protected  against  rats,  skunks, 
and  bears. 

Honeybees,  in  times  of  a  scarcity  of  pollen  and  honey,  may 
become  robbers,  ruthlessly  attacking  other  hives  and  carrying 
away  the  stores  contained  in  them. 

The  Diseases  of  Bees.  —  Bees  are  subject  to  several  important 
diseases.  Chief  among  these  are  European  foul  brood  and 
American  foul  brood  which  are  infectious  diseases  due  to 
bacteria.  These  microscopic  organisms  attack  the  eggs  and  the 
tissues  of  the  larvae.  The  diseases  may  spread  from  hive  to  hive 
throughout  the  apiary.  Dysentery  must  also  be  guarded  against. 
Improper  food  and  long  confinement  in  the  hive  are  mainly 
responsible  for  this  affliction. 


260  AN   INTRODUCTION  TO   ZOOLOGY 

2.  BEES  IN  GENERAL 

a.  Classification  of  Honeybees 

The  bees  belong,  with  the  ants,  wasps,  etc.,  to  the  order  Hy- 
menoptera.  In  this  order  are  included  all  insects  with  four 
membranous  wings,  the  hind  wings  being  the  smaller;  with 
biting  and  sucking  mouth  parts;  with  a  sting,  piercer,  or  saw  at 
the  end  of  the  abdomen  of  the  female;  and  with  a  complete 
metamorphosis,  i.e.  with  larval  and  pupal  stages  during  develop- 
ment. The  honeybee  belongs  in  the  family  Apidae,  and  is  the 
most  specialized  with  regard  to  its  communistic  life  of  any  of  the 
group.  The  species  of  honeybee  found  in  this  country  is  Apu 
mellifica.  A  number  of  other  species  of  honeybees  inhabiting 
Asia  and  Africa  are  placed  with  mellifica  in  the  genus  Apis. 
The  individuals  of  the  species  Apis  mellifica  are  not  all  alike  in 
structure,  color,  or  activities.  Seven  or  more  races  are  recognized. 
The  characteristics  of  the  more  important  races  are  contrasted 
in  Table  XII.  The  relations  of  the  honeybees  to  other  insects 
and  to  each  other  are  shown  in  outline  in  Table  XIII. 

b.  Gynandromorphs 

A  normal  colony  of  honeybees  contains,  as  stated  before,  a 
fertilized,  egg-laying  queen,  a  number  of  males  or  drones,  and 
thousands  of  sterile  females  or  workers.  A  number  of  bees  have 
been  discovered  which  showed  male  characters  in  certain  parts 
of  the  body  and  female  characters  in  other  parts.  Abnormal 
insects  of  this  kind  are  known  as  gynandromorphs.  Butterflies, 
ants,  and  bees  appear  to  be  more  often  afflicted  than  other 
insects.  The  best  well-known  instance  of  gynandromorphism 
occurred  in  a  hive  of  bees  at  Eugster  and  was  reported  by  von 
Siebolt.  This  hive  contained  an  Italian  queen  and  German 
drones.  The  workers  produced  by  this  queen  were  therefore 
hybrids.  Some  of  the  gynandromorphs  in  this  colony  had  the 
anterior  end  of  the  body  male,  the  posterior  female;  others 
exhibited  male  characters  on  the  right  and  female  on  the  left? 


THE  HONEYBEE  AND   BEES  IN  GENERAL 


261 


TABLE  XII 

SOME  OF  THE   CHARACTERISTICS   OF  THE   MORE   IMPORTANT  RACES  OF 
HONEYBEES l 


RACE 

COLOR  OF 
ABDOMEN 

DISPOSI- 
TION 

QUALITY 
AS  A  PRO- 
DUCER 

CAPPINGS 
OF  COMB 
HONEY 

REMARKS 

German 

Black 

Cross 

Poor 

White 

First   race   introduced 
into  America 

Italian 

Yellow 
stripes 

Gentle 

Best 

Fairly 
White 

Most  popular  race 

Carniolan 

Gray 

Gentle 

Good 

White 

Some  advocates  in  the 
United  States 

Caucasian 

Yellow 
Gray 

Gentlest 
Known 

Good 

White 

Recently    reintroduced 
Good  for  amateurs 

Banat 

Black 

Gentle 

Good 

White 

Recent 

Cyprian 

Yellow 

Vicious 

Good 

Watery 

Now   practically   aban- 
doned in  United  States. 

From  information  furnished  by  Dr.  E.  F.  Phillips. 


TABLE  XIII 

THE  RELATIONS  OF  THE  HONEYBEES  TO  OTHER  INSECTS  AND  TO  EACH 

OTHER 

CLASS  Insecta 

I 


ORDERS 

Diptera 
(Flies,  etc.) 

Lepidoptera 
(Butterflies 
and  Moths) 

Hym^r 

(Bees.  Wasp 

icptera  etc. 
s,  Ants,  etc.) 

FAMILIES 

Formicidae 
(Ants) 

Eumenidae 
(Solitary 
Wasps) 

Api 

(Be 

dae  etc 
es) 

GENERA 

Bombus 
(Bumblebee) 

Megachile 
(Leaf-cutter 
Bees) 

Al 

(Hone^ 

>is  etc. 
fbees) 

SPECIES 

Apis  indica 
(East  Indian  Honeybee) 

Apis  m 
(Common  " 

ellifica  etc. 
Honeybee) 

RACES 


Carniolans 


Italians 


Germans 


etc, 


262  AN  INTRODUCTION  TO  ZOOLOGY 

or  vice  versa;  still  others  had  male  and  female  characters  in 
different  parts  of  the  same  organ.  The  reproductive  organs 
were  often  partly  male  and  partly  female,  and  their  character 
could  not  be  determined  by  the  external  appearance  of  the 
gynandromorphs.  Various  explanations  have  been  offered  to 
account  for  this  peculiar  condition,  but  as  yet  the  data  necessary 
to  decide  the  question  have  not  been  furnished. 

c.  The  Relations  of  Bees  to  Other  Organisms 

Charles  Darwin  in  "  The  Origin  of  Species  "  has  used  the  bum- 
blebee to  illustrate  "  how  plants  and  animals,  remote  in  the  scale 
of  nature,  are  bound  together  by  a  web  of  complex  relations." 
He  found  "  that  the  visits  of  bees  are  necessary  for  the  fertiliza- 
tion of  some  kinds  of  clover;  for  instance,  twenty  heads  of  Dutch 
clover  (Trifolium  repens)  yielded  2290  seeds,  but  twenty  other 
heads,  protected  from  bees,  produced  not  one."  ..."  Humble- 
bees  alone  visit  red  clover,  as  other  bees  cannot  reach  the  nectar, 
—  hence  we  may  infer  as  highly  probable,  that,  if  the  whole 
genus  of  humblebees  became  extinct  or  very  rare  in  England,  the 
heart'sease  and  red  clover  would  become  very  rare,  or  wholly 
disappear.  The  number  of  humblebees  in  any  district  depends 
in  a  gieat  measure  upon  the  number  of  field  mice,  which  destroy 
their  combs  and  nests.  .  .  .  Now  the  number  of  mice  is  largely 
dependent,  as  every  one  knows,  on  the  number  of  cats.  .  .  . 
Hence  it  is  quite  credible  that  the  presence  of  a  feline  animal  in 
large  numbers  in  a  district  might  determine,  through  the  inter- 
vention first  of  mice  and  then  of  bees,  the  frequency  of  certain 
flowers  in  that  district!"  (236,  p.  65.)  The  influence  of  old 
maids  upon  the  number  of  cats  was  suggested  by  Huxley  as  an 
addition  to  Darwin's  illustration. 

Bees  and  Flowers.  —CROSS-POLLINATION.  —Bees  in  flying  from 
flower  to  flower  gathering  nectar  and  pollen  accomplish  what  is 
known  as  cross-pollination,  i.e.  the  pollen  from  one  flower  is 
carried  by  the  bee  to  another  flower.  Cross-pollination  seems  to 
be  of  advantage  to  the  seed,  since  many  flowers  are  structurally 


D 


FIG.  148.  Pollination  of  an  orchid  (Cypripedium)  by  a  bumblebee.  A,  bee  forcing  its 
way  into  the  flower  ;  B,  obtaining  nectar  within  the  flower  ;  C,  bee  escaping  brushes 
pollen  upon  the  stigma  of  the  flower;  before  finally  escaping  the  bee  receives  another 
load  of  pollen  from  the  anther.  (From  Coulter  after  Gibson.) 


THE  HONEYBEE  AND  BEES  IN  GENERAL  263 

constituted  so  as  to  prevent  self-pollination  and  the  visitations 
of  unsuitable  insects.  On  the  other  hand,  they  are  so  formed  as 
to  secure  visits  from  insects  that  fly  rapidly,  and  enter  many 
flowers,  thus  insuring  the  wide  distribution  of  pollen.  The  bees 
are  among  the  most  important  of  the  pollinizing  insects.  They 
are  especially  valuable  near  fruit  trees,  since  it  has  been  demon- 
strated that  orchards  containing  colonies  of  bees  are  more  pro- 
ductive than  neighboring  orchards  without  bees. 

Some  of  the  most  interesting  arrangements  for  the  securing  of 
cross-pollination  are  found  among  the  flowers  of  the  orchids. 
For  example,  the  common  lady-slipper  (Cypripedium)  is  adapted 
to  the  visitations  of  bumblebees.  The  flower  has  a  conspicuous 
pouch  with  an  opening  on  the  upper  side  of  the  inner  end,  over 
which  hangs  a  flap  possessing  two  anthers  and  a  stigma.  The 
bee  forces  its  way  into  the  pouch  (Fig.  148,  A)  and  sucks  up  the 
nectar  contained  within  (B).  In  escaping  from  the  pouch  it 
brushes  its  back  first  against  the  stigma  (C)  and  then  against  the 
anther  (D).  Any  pollen  present  upon  the  bee's  back  is  brushed 
off  upon  the  stigma  during  its  escape,  and  a  new  supply  is  then 
gathered  from  the  anther.  The  next  orchid  visited  receives  this 
pollen  upon  its  stigma,  and  adds  a  new  burden  to  the  bee's  load. 
The  bumblebee  thus  is  obliged  to  transfer  pollen  from  one  plant 
to  another  while  gathering  nectar  (203). 

THE  COLORS  OF  FLOWERS.  —  Many  scientists  believe  that  the 
brilliant  colors  of  many  flowers  attract  bees  and  other  insects, 
and  are  therefore  instrumental  in  causing  cross-pollination.  It  is 
further  claimed  that  the  bright  colors  themselves  are  the  result 
of  the  visits  of  insects,  since  those  flowers  that  happened  to  be 
more  brightly  colored  would  be  more  certain  to  attract  insects, 
and  therefore  more  liable  to  be  pollinated  and  produce  seed.  The 
selection  of  the  more  brightly  colored  flowers  for  a  sufficient 
number  of  years  would  result  in  the  survival  of  plants  which 
tend  to  produce  more  highly  colored  flowers.  This  entire  theory 
of  the  origin  of  the  colors  of  flowers  because  of  the  visits  of  in- 
sects seems  to  depend  upon  the  factor  that  attracts  the  insect  to 


264  AN  INTRODUCTION  TO  ZOOLOGY 

the  flower.  Certain  observations  apparently  prove  that  smell 
and  not  color  is  the  dominant  factor  (217),  whereas  other  ob- 
servations have  resulted  in  the  conclusion  that  insects,  such  as 
bees  and  butterflies,  that  show  a  high  degree  of  adaptation  to 
flowers,  prefer  red,  purple,  and  blue,  and  that  insects  poorly 
adapted  to  flowers  favor  yellow  and  white  (213).  Perhaps  the 
safest  view  to  adopt  at  present  is  that  color,  odor,  and  structural 
characters  are  all  important  factors  influencing  the  visits  of  bees 
and  other  insects  to  flowers  (204,  208). 

d.  The  Social  Life  of  Bees 

Certain  species  of  ants,  bees,  and  wasps  exhibit,  as  in  the  case 
of  the  honeybee,  remarkable  social  organizations.  How  this 
has  come  about  is  a  problem  not  yet  solved,  but  practically  all 
stages,  from  a  solitary  habit  to  a  complex  community,  are  illus- 
trated by  various  members  of  the  family  Apidae  (199,  207). 

(1)  A  SOLITARY  BEE.  —  The  leaf-cutter,  Megachile  acuta,  is 
a  solitary,  long-tongued  bee.     In  building  her  nest   she   digs  a 
tunnel,  usually  in  decayed  wood,  and  excavates  thimble-shaped 
cavities  in  the  bottom  of  it.     These  cavities  are  lined  with  pieces 
of  leaves,  generally  cut  from  a  rose  bush.     In  the  bottom,  the 
bee  places  a  quantity  of  pollen  and  nectar,  upon  which  she  lays 
an  egg.     She  then  plugs  the  entrance  with  pieces  of  leaves,  and 
flies  away.     The  young  that  hatch  from  her  eggs  live  upon  the 
stored  food. 

(2)  A  SOLITARY  BEE  THAT  WATCHES  ITS  YOUNG. — The  car- 
penter bee,  Ceratina  dupla,  makes  her  nest  by  digging  the  pith 
from  the  center  of  a  dead  twig  of  sumach  or  other  plant.     After 
a  long  tunnel  is  excavated  she  begins  at  the  bottom  and  constructs 
a  series  of  chambers  with  partitions  composed  of  pith.     At  the 
bottom  of  each  chamber  she  places  a  mass  of  pollen  and  lays 
an  egg.     She  then   waits   for   her  offspring  to  emerge.     "  The 
lower  one  hatches  first;   and,  after  it  has  attained  its  growth,  it 
tears  down  the  partition  above  it,  and  then  waits  patiently  for 
the  one  above  to  do  the  same.     Finally,  after  the  last  one  in  the 


THE  HONEYBEE  AND  BEES  IN  GENERAL 


265 


top  cell  has  matured,  the  mother  leads  forth  her  full-fledged 
family  in  a  flight  into  the  sunshine.  This  is  the  only  case  known 
to  the  writer  where  a  solitary  bee  watches  her  nest  till  her  young 
mature  "  (199,  p.  669). 

(3)  SOLITARY  BEES  WITH  A  TENDENCY  TOWARD  A  GREGARIOUS 

HABIT. — Short-tongued  bees  of  the  genus  Andrena  are  called 

mining  bees,  because  they  dig  tunnels  in  the  earth,  often  more 

than  a  foot  deep  (Fig,  149,  B).     From  the  sides  of  these  tunnels 

A 


FIG.  149.  Diagrams  of  nest  burrows  of  short-tongued  mining  bees.     A,  nest 
of  Halictus;  B,  nest  of  Andrena.     (From  Kellogg.) 

branches  lead  into  cells,  in  each  of  which  pollen  is  stored  and 
an  egg  is  laid.  The  entrance  to  the  cell  is  then  closed.  These 
mining  bees  seem  to  enjoy  the  company  of  others  of  their  kind, 
and  though  each  digs  her  own  nest,  many  tunnels  may  be  placed 
close  to  one  another,  forming  villages,  sometimes  as  much  as  fif- 
teen feet  in  diameter,  and  containing  over  a  thousand  nests. 


266  AN  INTRODUCTION  TO  ZOOLOGY 

(4)  SOLITARY  BEES  WITH  A  TENDENCY  TOWARD  COMMUNITY 
LIFE.  — The  mining  bees  of  the  genus  Halictus  make  burrows  in 
sand  banks  and  the  sides  of  cliffs  (Fig.  149,  A).     "A  remarkable 
feature  in  the  habits  of  the  bees  of  this  genus  is  that  several 
females  unite  in  making  a  burrow  into  a  bank,  after  which  each 
female  makes  passages  extending  sidewise  from  this  main  burrow 
or  public  corridor  to  her  own  cells.     While  Andrena  builds  vil- 
lages composed  of  individual  homes,  Halictus  makes  cities  com- 
posed of  apartment  houses  "  (199,  p.  666). 

(5)  SOCIAL  BEES  WITH  ANNUAL  COLONIES.  — The  bumblebees 
of  the  genus  Bombus  form  communities  composed  of  queens,  males, 
and  workers.     Only  young  fertilized  queens  survive  the  winter. 
In  the  spring  each  selects  an  old  deserted  field-mouse  nest  and 
starts  a  colony.     First  a  number  of  workers  are  reared,  being 
cared  for  by  their  mother;    these  workers  then  take  over  the 
household  duties,  and  the  queen  devotes  herself  to  laying  eggs. 
As  autumn  approaches  males  and  queens  appear,  and  finally  all 
perish  except  the  young  queens. 

(6)  SOCIAL  BEES  WITH  PERMANENT  COLONIAL  LIFE. — The 
honeybee  differs  from  the  bumblebee  in  many  ways.     Its  life  ac- 
tivities are  more  complex,  and  its  colonies  are  able  to  pass  the 
winter  without  perishing.     That  their  complex  community  life 
had  evolved  from  a  solitary  condition  through  the  stages  men- 
tioned above  should  not  be  understood,  but  the  life  histories  of 
the  solitary  bees  and  the  bumblebee  show  many  gradations  be- 
tween the  strictly  solitary  life  and  the  complex  social  lives  of 
these  remarkable  insects. 


PIG.  150.  Aristotle,  384-322  B.C.    (From  Locy.) 


CHAPTER  XIII 
HISTORICAL  ZOOLOGY 

No  one  knows  when  man  began  to  study  animal  life.  The 
pursuit  of  certain  forms  for  food,  the  domestication  of  others, 
and  the  practice  of  animal  sacrifice  doubtless  furnished  some  crude 
and  scattered  notions  of  anatomy,  physiology,  and  ecology,  even 
in  remote  antiquity.  The  first  scientific  treatises  that  had  an 
influence  upon  modern  zoological  ideas  were  not  written  until 
about  three  hundred  and  fifty  years  before  Christ.  At  this  time 
Aristotle's  works  appeared,  and  so  careful  were  the  observations 
of  this  remarkable  man  that  they  were  considered  authoritative 
for  twenty  centuries. 

It  is  convenient  to  divide  zoological  history  into  five  periods: 
(i)  the  Greek  Period,  (2)  the  Roman  Period,  (3)  the  Period  of  the 
Middle  Ages,  (4)  the  Encyclopedic  Period,  and  (5)  the  Modern 
Period. 

i.  THE  GREEK  PERIOD 

Aristotle  (384-322  B.C.,  Fig.  150)  was  the  foremost  pupil  of 
Plato  and  the  tutor  of  Alexander  the  Great.  He  was  early  left 
an  orphan  with  a  considerable  fortune,  and  devoted  his  life  to 
study  in  a  variety  of  fields.  He  published  three  hundred  works 
on  philosophy,  psychology,  rhetoric,  and  other  subjects,  but  his 
most  important  contributions  were  to  natural  history,  of  which 
science  he  is  justly  called  the  "  father."  He  knew  over  five 
hundred  species  of  vertebrates  and  many  invertebrates,  and  at- 
tempted to  classify  them.  His  greatest  works  were  on  the  natural 
h;  story  of  animals,  the  parts  of  animals,  and  the  development  of 

267 


268  AN  INTRODUCTION  TO  ZOOLOGY 

animals.  They  reveal  a  remarkable  familiarity  with  the  facts  of 
comparative  anatomy,  physiology,  and  embryology.  Aristotle's 
ideas  later  furnished  the  starting  point  for  the  founding  of  our 
modern  systems  of  classification  and  theories  of  evolution,  but 
his  greatest  contributions  to  zoology  were  the  methods  of  work 
which  he  introduced.  He  was  a  critical  compiler,  and,  from  the 
fabric  of  scattered  facts  and  fancies  which  existed  at  his  time, 
produced  a  compact  and  fairly  accurate  account  of  animals.  He 
was  not  content,  however,  to  accept  old  statements,  but  verified 
everything  by  careful  examinations  of  the  animals  themselves, 
and  added  many  new  facts. 

2.  THE  ROMAN  PERIOD 

Pliny  (23-79  A-D0  l£d  an  active  public  life  under  the  Roman 
Empire  as  a  naval  commander.  His  writings  consist  of  thirty- 
seven  volumes,  which  had  a  great  influence  on  the  ideas  of  natu- 
ralists during  succeeding  centuries.  Unfortunately,  they  are  not 
critical,  combining  fact,  fable,  and  fancy  in  accounts  of  dragons, 
gorgons,  and  other  imaginary  monsters.  As  a  whole,  Pliny's 
influence  was  detrimental  to  zoological  progress,  and  helped 
inaugurate  an  era  of  superstition. 

Claudius  Galen  (130-200  A.D.)  was  a  Greek  physician  who 
practiced  for  a  time  in  Rome.  He  was  the  greatest  anatomist  of 
antiquity,  and  his  writings  remained  the  best  on  the  subject  until 
the  sixteenth  century.  These  works  were  the  results  of  his  own 
careful  studies  and  dissections  of  the  higher  animals,  and  his 
descriptions  were  remarkably  clear  and  forceful. 

3.  MIDDLE  AGES 

The  Middle  Ages  are  a  blank,  so  far  as  zoological  progress  is 
concerned.  Superstition  was  rampant,  and  the  belief  in  various 
fabled  animals  was  prevalent.  All  zoological  questions  were 
referred  to  the  ancient  authorities,  and  original  investigation  was 
at  a  standstill.  In  one  controversy  a  series  of  papers  was  pub- 


FIG.  151.  Linnaeus  at  Sixty,  1707-1778.     (From  Locy.) 


HISTORICAL  ZOOLOGY  269 

lished  with  respect  to  the  number  of  teeth  in  a  horse's  mouth. 
In  this  instance  not  one  of  the  writers  seems  to  have  thought  of 
examining  an  animal,  but  all  were  satisfied  to  quote  the  words  of 
men  who  had  died  centuries  before.  This  intellectual  stagnation 
was  primarily  due  to  the  fact  that  all  learning  was  in  the  hands  of 
the  Church,  and  nothing  was  considered  important  except  matters 
pertaining  to  religion. 

4.  ENCYCLOPEDIC  PERIOD 

Conrad  Gesner  (1516-1565  A.D.)  may  be  mentioned  as  one 
of  the  best  examples  of  this  active  but  uncritical  period.  He 
wrote  many  works,  and  his  natural  history  (Historia  Animalium) 
was  the  best  work  on  zoology  for  a  long  time.  The  activities  of 
the  naturalists  of  this  period  foreshadowed  the  awakening  of 
ideas  which  were  to  throw  off  the  respect  for  authoritative  writ- 
ings that  had  hampered  the  scholars  of  the  Middle  Ages. 

5.  MODERN  PERIOD 

Before  the  intellectual  awakening  of  the  sixteenth  century, 
naturalists  essayed  to  cover  the  entire  field  of  zoological  sciences. 
The  workers  of  the  Modern  Period,  however,  have  confined  them- 
selves to  more  limited  fields,  and  certain  individuals  are  respon- 
sible in  large  part  for  the  development  of  the  various  subsciences 
defined  in  Chapter  I.  On  this  account  the  subjects  of  systematic 
zoology,  comparative  anatomy,  histology,  embryology,  physiol- 
ogy, and  evolutionary  zoology  are  considered  separately  in  the 
following  pages. 

a.  Systematic  Zoology 

Before  the  time  of  John  Ray  (1629-1705)  there  had  been  no 
very  definite  idea  of  a  species  as  such.  Ray  originated  the  modern 
idea  of  a  species,  and  defined  it  as  the  offspring  of  similar  parents. 
He  published  several  lists  of  careful  descriptions  of  the  species 


270  AN  INTRODUCTION  TO  ZOOLOGY 

with  which  he  was  familiar,  together  with  a  system  of  classifica- 
tion. Thus  the  way  was  cleared  for  the  greatest  worker  in  this 
field,  Carl  Linnaeus  (1707-1778,  Fig.  151),  who  attempted  to 
describe  all  the  existing  species  of  animals  and  plants.  He 
succeeded  in  listing  4378  in  the  tenth  edition  of  his  greatest  work, 
Systema  Natures.  His  great  influence,  and  the  wide  recognition 
which  was  accorded  his  work,  made  the  systematic  side  of  zoology 
the  most  active  field  of  investigation  for  a  long  time  after  his 
death.  The  aim  of  the  systematic  zoologist  has  been  to  describe 
all  the  species  of  animals,  and  to  arrange  them  according  to  a 
natural  system,  i.e.  a  system  that  will  show  their  true  relation- 
ships to  one  another. 

b.  Comparative  Analomy 

Anatomy  up  to  the  sixteenth  century  consisted  in  descriptions 
of  the  structure  of  single  animals.  The  points  of  resemblance  of 
different  animals  finally  led  zoologists  to  compare  the  anatomy 
of  one  with  another.  The  French  scientist  Cuvier  (1769-1832, 
Fig.  152),  may,  however,  be  considered  the  founder  of  this  branch 
of  zoology,  since  he  extended  his  studies  over  the  entire  animal 
kingdom,  and  added  a  great  mass  of  personal  observations  to  the 
many  descriptions  published  by  his  predecessors.  Besides  a 
number  of  treatises  on  comparative  anatomy,  he  wrote  a  book  on 
the  fossil  remains  of  animals  which  founded  the  science  of  verte- 
brate paleontology. 

Among  Cuvier's  more  noted  successors  were  the  Englishmen, 
Richard  Owen  (1804-1892)  and  Thomas  H.  Huxley  (1825-1895), 
and  the  American,  E.  D.  Cope  (1840-1897).  To  Richard  Owen 
we  owe  the  introduction  of  the  ideas  of  analogy  and  homology. 
His  work  on  the  comparative  anatomy  of  vertebrates  has  been  of 
great  service  ever  since  its  publication.  Although  Huxley 
made  many  investigations  and  published  a  number  of  papers  on 
the  comparative  anatomy  of  animals,  he  is  best  known  because 
of  his  influence  in  popularizing  zoology.  Cope  played  an  impor- 


FIG.  152.  Cuvier,  1769-1832.     (FromLocy.) 


FIG.  153.  William  Harvey,  1578-1667.     (FromLocy.) 


HISTORICAL  ZOOLOGY  271 

tant  role  in  the  advancement  of  his  chosen  field  of  work.  He  not 
only  studied  living  forms,  but  was  one  of  the  greatest  contribu- 
tors to  the  science  of  paleontology. 

c.  Histology 

Bichat  (1771-1801)  was  the  founder  of  histology.  His  work 
was  of  a  high  order,  ranking  with  that  of  the  physiologist  J. 
Muller  and  the  embryologists  Van  Baer  and  Balfour.  He 
directed  the  attention  of  biologists  to  the  study  of  tissues,  and, 
though  he  did  not  use  a  microscope  and  failed  to  make  out  the 
cellular  structures,  his  investigations  led  to  the  modern  science 
of  histology. 

The  introduction  of  the  microscope  added  new  impetus  to  the 
science  of  the  anatomy  of  tissues,  and  resulted  in  the  announce- 
ment of  the  cell  theory  by  Schwann  in  1839,  as  described  in 
Chapter  III  (pp.  35-36).  Max  Schultze  (1825-1874),  by  identi- 
fying vegetable  protoplasm  with  animal  sarcode,  reformed  the 
ideas  regarding  cells.  Rudolph  Virchow  (1821-1903)  took  up 
the  study  of  diseased  tissues,  and  helped  establish  the  cell  theory 
as  it  is  understood  to-day. 

d.  Embryology 

The  rise  of  embryology  dates  from  the  time  of  William  Harvey 
(1578-1667,  Fig.  153)  and  Marcello  Malpighi  (1628-1694).  The 
former  published  in  1651  a  work  descriptive  of  the  embryology  of 
the  chick,  and  of  several  mammals.  Malpighi  was  further  re- 
moved from  the  influence  of  the  ancients  than  Harvey,  and  his 
works  on  embryology  are  of  greater  value.  His  contributions 
include  a  detailed  description  of  the  development  of  the  chick, 
illustrated  by  excellent  drawings. 

Previous  to  the  time  of  Friedrich  K.  Wolff,  embryologists  be- 
lieved in  what  is  known  as  the  preformation  theory.  According 
to  this  theory  the  embryo  exists  in  miniature  within  the  egg, 
and  during  development  simply  unfolds  and  expands.  From 


272  AN  INTRODUCTION  TO  ZOOLOGY 

this  theory  grew  the  idea  of  encasement,  i.e.  the  preformed  em- 
bryo must  contain  a  second  smaller  miniature,  and  this  another 
ad  infinitum,  just  as  a  series  of  boxes  may  be  made  to  fit  within 
one  another.  Wolff  did  more  than  any  other  embryologist  to 
overthrow  the  preformation  theory  and  to  introduce  in  its  stead 
the  idea  of  epigenesis.  By  epigenesis  Wolff  meant  the  gradual 
formation  of  organs  from  an  unorganized  egg.-  The  establish- 
ment of  this  theory  changed  all  subsequent  work  in  embryology. 
Karl  E.  von  Baer  (1792-1876,  Fig.  154)  was  the  greatest  of  all 
embryologists.  His  most  important  services  were  (i)  the  excel- 
lence of  his  work,  which  raised  the  standard  of  embryological 
investigations,  (2)  the  establishment  of  the  germ-layer  theory, 
and  (3)  the  broadening  of  the  field  of  embryology  by  making  it 
comparative.  He  is  now  known  as  the  "  father  of  modern  em- 
bryology." Francis  M.  Balfour  (1851-1882)  in  1880-1881  pub- 
lished his  Comparative  Embryology.  This  work  is  a  summary  of 
the  enormous  literature  on  the  subject  that  had  accumulated 
during  the  period  inaugurated  by  von  Baer.  It  also  contains  the 
result  of  much  original  research  and  many  broad  generalizations. 

e.  Physiology 

The  medical  men  of  ancient  times  depended  largely  upon  the 
investigations  of  Galen.  Diseases  were  supposed  to  be  due  to 
the  presence  of  spirits  and  humors  in  the  body.  This  idea,  called 
the  pneuma-theory,  was  not  overthrown  until  the  Revival  of 
Learning.  The  names  of  William  Harvey  (1578-1667),  Albrecht 
Haller  (1708-1777),  and  Johannes  Miiller  (1801-1858)  are  among 
the  most  famous  in  the  field  of  physiology.  Harvey  demon- 
strated the  connections  between  arteries  and  veins,  and  discov- 
ered the  circulation  of  the  blood.  Although  these  contributions 
to  knowledge  have  made  his  name  famous,  the  introduction  by 
him  of  experimental  methods  in  physiological  investigations  have 
had  a  more  profound  influence  upon  the  progress  of  physiology. 

Haller  separated  the  study  of  physiology  from  medicine  and 


FIG.  154.  Karl  Ernst  von  Baer,  1792-1876.     (From  Locy.) 


FIG.  155.  Johannes  Miiller,  1801-1858.  (From  Locy.) 


HISTORICAL  ZOOLOGY 


273 


anatomy,  and,  in  his  Elements  of  Physiology  (1758),  summed  up 
the  principal  facts  and  theories  of  his  predecessors.  Johannes 
Muller  (Fig.  155)  founded  modern  comparative  physiology,  and 
prepared  a  handbook  of  the  physiology  of  man,  based  upon  the 
personally  verified  statements  of  others  and  upon  his  own  obser- 
vations, which  to  this  day  has  no  equal.  He  made  use  of  the 
microscope,  and  brought  to  his  work  a  knowledge  of  physics, 
chemistry,  and  psychology.  Since  his  time  physiological  in- 
vestigations have  progressed  along  physical  and  chemical  lines, 
and  vital  activities  are  now  explained  by  many  in  physico- 
chemical  terms. 


/.  Evolutionary  Zoology 

It  is  difficult  in  this  place  to  give  an  adequate  history  of  evolu- 
tionary zoology  without  discussing  the  evolution  theory  in 
detail.  We  shall,  however,  leave  that  for  the  succeeding  chap- 
ter, and  restrict  ourselves  to  a  brief  account  of  the  work  of  a  very 
few  men  who  have  accomplished  the  most  in  this  field.  Lamarck 
(1774-1829)  was  one  of  the  first  to  recognize  the  instability  of 
species,  and  was  the  first  to  make  use  of  a  genealogical  tree  to 
show  the  relationships  of  animals.  His  most  important  work  is 
entitled  Philosophic  Zoologique,  published  in  1809.  It  contains 
statements  of  his  belief  in  the  inheritance  of  acquired  characters, 
the  result  of  use  and  disuse,  and  other  less  important  views. 

But  the  greatest  of  all  scientists  who  influence  our  evolutionary 
ideas  was  Charles  Darwin  (1809-1882,  Fig.  156).  His  book  on 
the  origin  of  species,  published  in  1859,  changed  the  trend  of 
investigations  in  many  fields  of  science,  and  did  more  than  any 
other  factor  to  place  evolution  upon  a  firm  foundation.  At  the 
present  time  organic  evolution,  or  the  transmutation  of  species, 
is  accepted  by  practically  every  well-known  zoologist,  and  our 
attention  is  directed  toward  the  problem  of  the  method  by  which 
evolution  takes  place.  ,  Darwin's  theories  have  been  assailed 
frequently,  and  are  no  more  accepted  by  many  zoologists  in 


274  AN  INTRODUCTION  TO  ZOOLOGY 

their  original  form,  though  they  continue  to  exert  an  important 
influence  upon  many  lines  of  research. 

Some  of  the  more  recent  workers  in  this  field  are  Mendel, 
Weismann,  and  de  Vries.  Gregor  Mendel  (1822-1884)  was  an 
Austrian  monk  whose  results  from  a  study  of  the  crossing  of 
different  kinds  of  peas  and  other  plants  have  given  us  one  of  the 
few  laws  of  heredity.  An  account  of  his  work  will  be  found  in 
Chapter  XIV  (p.  289).  August  Weismann  (born  1834)  is  an 
ardent  supporter  of  Darwinism.  He  is  the  foremost  living  evo- 
lutionary zoologist.  Hugo  de  Vries  has  recently  brought  forth 
a  work  entitled  Die  Mutationstheorie,  published  in  1901,  which 
combats  Darwin's  theories  of  the  origin  of  species,  and  offers 
in  its  stead  the  "  mutation  theory,"  a  discussion  of  which  is 
reserved  for  the  next  chapter. 

g.   Zoology  of  To-day 

As  the  facts  of  zoological  sciences  have  increased  in  number, 
the  fields  of  work  have  become  more  numerous  and  narrower,  and 
investigations  are  now  carried  on  by  more  improved  methods  and 
in  greater  detail  than  ever  before.  Morphological  studies  are 
being  supplemented  by  experimental  investigations  in  embryol- 
ogy, regeneration,  heredity,  evolution,  and  other  sciences.  Ani- 
mal behavior  is  one  of  the  most  favored  subjects  of  research,  and 
is  rapidly  leading  psychologists  to  a  better  understanding  of  the 
animal  mind. 

Zoology  at  the  present  time  presents  a  rich  field  for  original 
research.  Many  of  the  apparently  simple  "  laws  "  have  been 
found  on  close  examination  to  be  really  very  complex.  All  lines 
of  experimental  work  offer  large  rewards  for  the  student  and  open 
up  for  him  a  countless  number  of  fascinating  problems. 


FIG.  156.  Charles  Darwin,  1809-1882.     (From  Locy.) 


CHAPTER  XIV 

GENERAL   CONSIDERATIONS    OF   ZOOLOGICAL   FACTS 
AND   THEORIES1 

i.  HEREDITY  AND  EVOLUTION 

a.  Facts 
(i)  The  Distribution  of  Animals  in  Space — Zoogeography 

THE  earth's  surface  has  an  area  of  about  two  hundred  million 
square  miles,  five  eighths  of  which  is  covered  by  the  sea.  This 
vast  territory  is  not  uniform,  but  presents  a  great  number 
of  sets  of  conditions.  The  major  habitats  are  the  solid  earth, 
the  liquids  upon  the  earth,  and  the  atmosphere.  Since  proto- 
plasm is  impressionable  and  retains  impressions,  organisms  are 
modified  by  and  adjusted  to  these  different  conditions;  e.g. 
Paramecia  in  water  of  a  certain  kind,  the  earthworm  in  ground 
that  is  not  too  sandy,  and  the  honeybee  in  the  air  near  flowering 
plants.  The  facts  of  zoogeography  have  led  to  the  formulation 
of  the  three  following  laws:  (i)  the  law  of  definite  habitats, 
(2)  the  law  of  dispersion,  and  (3)  the  law  of  barriers  and  highways. 

THE  LAW  OF  DEFINITE  HABITATS.  —  Among  the  most  impor- 
tant physical  factors  that  determine  the  habitat  of  an  animal  are 
temperature,  water,  density,  light,  molar  agents,  and  food.  The 

1  The  author  realizes  the  difficulty  of  doing  justice  to  the  facts  and  theories 
of  evolution  and  heredity,  the  social  life  of  animals,  reflexes,  instincts,  and 
the  animal  mind  in  one  short  chapter  ;  nevertheless,  he  has  endeavored  to 
-  give  a  concise  account  of  these  subjects,  believing  that  the  majority  of 
students  who  take  the  introductory  course  in  biology  do  not  continue  with 
the  more  advanced  courses  and  therefore  have  no  other  opportunity  of  be- 
coming acquainted  with  this  important  phase  of  zoology.  No  doubt  in 
many  cases  the  aid  of  a  teacher  will  be  necessary  for  a  clear  understanding 
of  the  subjects  treated. 

275 


276  AN  INTRODUCTION  TO  ZOOLOGY 

continent  of  North  America  has  been  divided  by  scientists  into 
definite  regions,  according  to  the  sum  total  of  the  temperature 
during  the  season  of  growth ;  and  regions  of  a  certain  temperature, 
though  widely  separated,  are  liable  to  support  similar  faunas 
(255).  Winter  is  met  by  northern  animals  in  one  of  four  ways : 
(i)  by  dying,  e.g.  adult  butterflies,  (2)  migrating,  e.g.  birds, 
(3)  hibernating,  e.g.  bears,  (4)  remaining  active,  e.g.  rabbits. 
Animals  living  in  tropical  regions  pass  the  summer  in  many  case? 
in  a  torpid  condition,  and  are  said  to  be  aestivating. 

A  certain  amount  of  water  is  necessary  for  life,  as  the  bodies 
of  animals  are  made  up  of  from  55  to  95  per  cent  water.  In  dry 
climates  animals  have  thick  skins,  and  thus  evaporation  is  pre- 
vented. Aquatic  animals  obtain  air  from  water,  as  do  also 
some  terrestrial  species,  e.g.  the  earthworm.  The  density  of 
the  water  and  its  salinity  determine  the  distribution  of  many 
aquatic  organisms. 

Light,  as  we  have  seen,  plays  a  leading  role  in  the  lives  of  ani- 
mals; many  species  require  it  (Euglena),  but  others  shun  it  as 
much  as  possible  (crayfish),  principally  in  order  to  escape  their 
enemies.  Molar  agents,  such  as  currents,  limit  the  number  of 
species  and  individuals,  especially  where  they  act  with  much 
force.  And  finally,  food  conditions  are  most  effective,  since 
carnivorous  animals,  e.g.  lions,  must  live  where  they  may  obtain 
flesh,  herbivorous  animals,  e.g.  deer,  must  live  where  suitable 
vegetation  abounds,  and  omnivorous  animals,  e.g.  crayfishes, 
where  both  flesh  and  vegetation  of  certain  sorts  exist.  Table 
XIV  presents  roughly  the  four  principal  kinds  of  fauna,  and  their 
modes  of  existence. 

The  general  statement  may  be  made  that  the  major  habitats 
are  broken  up  into  minor  habitats  by  variations  in  the  conditions, 
and  the  constitution  of  the  organism.  There  are  great  differ-- 
ences  in  the  exactness  with  which  the  different  forms  are  confined 
to  different  sets  of  environmental  conditions.  This  will  be 
better  understood  after  the  other  laws  of  distribution  have  been 
discussed. 


GENERAL  ZOOLOGICAL   FACTS  AND  THEORIES        277 


TABLE  XIV 

THE   FOUR  KINDS   OF   FAUNA   AND  THEIR  MODES   OF   EXISTENCE 


FAUNA 

HABITAT 

FACTORS  OF  HABITAT 

EXAMPLES 

Halobios 

Marine 

{Density 
Darkness 
Molar  agents 

Lobster 
Whale 

Grantia 

Limnobios 

Fresh  water 

f  Density 
Water  I  Darkness 
[  Molar  agents 

Crayfish 
Paramecium 
Hydra 
Planaria 

Geobios 

Aerial 
Terrestrial 

Subterrestrial 

Dry,  light,  low  specific  gravity 
Thigmotaxis,  darkness 

{Honeybee 
Birds 
Squirrels 
j  Mole 
[  Earthworm 

Biobios 

Parasitic 

Fluid  food,  darkness 

A  scar  is 
Plasmodium 

THE  LAW  OF  DISPERSION.  —  Animals  tend  to  migrate  from  the 
region  of  their  origin.  The  reason  for  this  is  in  dispute  at  the 
present  time.  It  has  been  held  for  many  years  that  every  animal 
produces  a  greater  number  of  offspring  than  can  be  supported 
in  its  particular  habitat,  and,  since  parents  and  offspring  cannot 
occupy  the  same  area,  some  individuals  must  either  migrate  or 
die.  There  are  many  scientists,  on  the  other  hand,  who  believe 
that  no  overcrowding  takes  place,  but  that  dispersion  is  the 
result  of  a  search  for  food. 

Each  species  of  animal  is  supposed  to  have  originated  in  a  defi- 
nite region  of  the  earth's  surface,  and  to  have  migrated  in  various 
directions,  enlarging  its  habitat  year  by  year  until  an  approxi- 


278  AN  INTRODUCTION  TO  ZOOLOGY 

mately  permanent  area  is  occupied.  The  region  from  which 
migration  took  place  is  termed  the  center  of  dispersal.  The 
following  criteria  have  been  given  to  determine  the  center  of  dis- 
persal of  a  species:  the  abundance  and  size  of  individuals,  the 
location  of  closely  related  forms,  and  the  migration  routes  now 
selected  by  the  species  (225). 

THE  LAW  OF  BARRIERS  AND  HIGHWAYS.  — Animals  are  confined 
to  certain  habitats  by  barriers.  They  are  prevented  from  gaining 
access  to  a  new  region  by  the  change  in  the  media,  by  dearth  of 
food,  and  the  interference  of  other  animals.  Common  barriers 
are  mountains,  bodies  of  water,  open  country  for  forest  animals, 
and  forests  for  prairie-inhabiting  species.  For  example,  the  cray- 
fish, Cambarus,  migrates  up  and  down  streams,  but  cannot  travel 
overland  to  neighboring  streams,  the  honeybee  cannot  fly  across 
the  ocean,  nor  can  Hydra  enter  the  sea. 

The  reverse  of  a  barrier  is  a  highway.  Apparently  there  are 
routes  of  migration  which  are  especially  favored.  This  may  be 
illustrated  by  the  flight  of  birds  southward  in  the  autumn,  and 
northward  again  the  following  spring.  Many  birds  migrate 
up  and  down  the  Mississippi  Valley,  and  along  the  Atlantic 
coastal  plain. 

COSMOPOLITAN  GROUPS  OF  ANIMALS.  —  Some  species  of  animals 
have  wide  ranges,  e.g.  some  are  found  inhabiting  practically 
every  large  land  area  on  the  earth's  surface.  Sixteen  families 
of  birds,  including  doves,  owls,  and  ducks,  and  one  family  of 
mammals,  the  bats,  are  cosmopolitan  groups.  Doubtless  the 
wings  of  the  birds  have  facilitated  their  dispersal,  since  they  give 
them  remarkable  powers  of  locomotion. 

RESTRICTED  GROUPS  OF  ANIMALS. — In  a  number  of  cases  certain 
species  are  restricted  to  very  limited  areas.  The  mountain  goat 
is  found  only  in  the  higher  Rocky  and  Cascade  mountains  to 
Alaska.  Islands  are  famous  for  the  presence  of  restricted  species. 
Darwin's  descriptions  of  the  animals  he  found  in  the  Galapagos 
Islands  read  like  fairy  tales  (235). 

DISCONTINUOUS  DISTRIBUTION.  — Whenever  a  species  occurs  in 


GENERAL  ZOOLOGICAL   FACTS  AND  THEORIES        279 

two  widely  separated  regions,  it  is  safe  to  conclude  that  the  dis- 
tribution must  once  have  been  continuous.  Examples  of  dis- 
continuously  distributed  animals  are  rare.  Tapirs  inhabit  tropi- 
cal America  and  the  Malay  Archipelago;  the  reed  bunting 
in  England  reappears  in  Japan;  the  white  mountain  butterfly 
inhabits  the  Rocky  Mountains  of  British  Columbia  and  the 
region  of  Hudson's  Bay,  but  is  absent  between  these  localities. 

GENERAL  CONCLUSIONS. —  "  The  laws  governing  the  distribution 
of  animals  are  reducible  to  three  very  simple  propositions.  Every 
species  of  animal  is  found  in  every  part  of  the  earth  having  con- 
ditions suitable  for  its  maintenance,  unless:  — 

"  (a)  Its  individuals  have  been  unable  to  reach  this  region, 
through  barriers  of  some  sort;  or 

"  (b)  Having  reached  it,  the  species  is  unable  to  maintain  itself, 
through  lack  of  capacity  for  adaptation,  through  severity  of 
competition  with  other  forms,  or.  through  destructive  conditions 
of  environment;  or 

"  (c)  Having  entered  and  maintained  itself,  it  has  become  so 
altered  in  the  process  of  adaptation  [or  as  a  result  of  other  pro- 
cesses] as  to  become  a  species  distinct  from  the  original  type  " 
(249,  p.  314). 

(2)  The  Distribution  of  Animals  in  Time 

The  fossil  remains  of  animals  that  lived  millions  of  years  ago 
give  us  authentic  records  of  the  fauna  present  upon  the  earth's 
surface  at  that  time.  These  records,  unfortunately,  are  frag- 
mentary, since  only  the  hard  parts  of  the  animals  were  preserved, 
and  these,  when  discovered,  are  almost  always  broken  and  in- 
complete, making  the  reconstruction  of  many  parts  necessary. 
The  number  of  species  of  fossil  forms  known  at  the  present 
time  is  given  in  parentheses  after  the  descriptions  of  the  phyla 
of  the  animal  kingdom  in  Chapter  I  (p.  5).  From  the  evidence 
obtained  from  fossils,  paleozoologists  have  constructed  a  table 
(Table  XV)  showing  the  geological  periods,  arranged  in  the 


280 


AN  INTRODUCTION   TO  ZOOLOGY 


order  of  their  succession,  and  the  time  of  origin  and  relative 
number  of  the  different  groups  of  animals  (279). 


TABLE  XV 


THE    GEOLOGICAL    PERIODS    AND    THE    ORIGIN    AND    RELATIVE    NUMBER    OF 
THE   DIFFERENT    GROUPS    OF    ANIMALS 


ERA 


PERIOD 


i 


2 

£     E     * 

s      S      < 


Cenozoic 


Recent 

Pleistocene 

Pliocene 

Miocene 

Eocene 


r  10 


3,000,000 


Mesozoic 


Cretaceous 

Jurassic 

Triassic 


7,200,000 


Paleozoic 


Permian 

Carboniferous 

Devonian 

Silurian 

Cambrian 


>f  17,500,000 


Archaean 


Laurentian 


(3)  Variations  in  Animals 

We  have  already  noted  (p.  79)  that  no  two  organisms  are  ever 
exactly  alike.  In  other  words,  individuals  of  different  species 
are  so  dissimilar  as  to  be  in  most  cases  easily  recognized.  Indi- 


GENERAL  ZOOLOGICAL   FACTS  AND    THEORIES        281 

viduals  of  the  same  species,  if  examined  closely,  will  also  be  found 
to  vary  in  certain  respects;  and  even  the  offspring  of  the  same 
parents  differ  from  one  another  to  a  greater  or  less  degree. 
The  difference  between  child  and  parent  and  between  children  is 
called  variation.  Variations  are  important  in  any  discussion  of 
evolution  and  heredity,  since  their  origin  and  influence  upon  the 
transmutation  of  species  constitute  the  fundamental  problems 
that  are  occupying  the  attention  of  philosophical  zoologists  at 
the  present  time. 

CONTINUOUS  OR  FLUCTUATING  VARIATIONS.  — If  a  hundred  men 
are  gathered  at  random,  and  arranged  in  a  row  according  to  their 
heights,  it  will  be  found  that  they  decrease  gradually  from  the 
tallest  at  one  end  to  the  shortest  at  the  other.  This  is  an  illus- 
tration of  continuous  variation.  If  now  we  examine  this  line 
more  closely,  we  will  find  that  the  greatest  number  measure  about 
sixty-five  inches  in  height,  and  that  the  number  of  any  height 
decreases  as  the  ends  of  the  line  are  approached.  The  measure- 
ments decrease  or  increase  gradually  from  the  central  mean  to 
one  end  or  the  other,  and  as  many  variations  will  be  found  above 
the  mean  as  below.  The  offspring  of  any  of  the  individuals 
selected  will  show  a  variability  that  is  similarly  fluctuating,  and 
follows  the  law  of  probabilities.  This  law  holds  that  slight 
modifications  from  the  mean  are  most  abundant,  and  that  the 
greater  the  variation,  and  the  nearer  the  extremes  are  approached, 
the  less  numerous  the  modifications  become. 

DISCONTINUOUS  VARIATIONS.  —  Animals  known  as  "  sports," 
"  saltations,"  or  "  mutations "  are  examples  of  discontinuous 
variability.  These  differ  from  continuous  variations  in  that  they 
are  able  to  transmit  their  differences  from  the  mean  to  their  de- 
scendants. Some  of  the  most  noted  sudden  variations  are  the 
hornless  cattle  of  Paraguay,  the  niata  breed  of  oxen  of  South 
America,  and  the  mauchamp  and  ancon  sheep.  The  last-named 
originated  from  a  ram  with  crooked  legs  and  a  long  back,  that  was 
born  in  Massachusetts  in  1791.  When  crossed  with  common 
sheep,  his  peculiar  characteristics  reappeared  in  the  offspring, 


282  AN  INTRODUCTION  TO  ZOOLOGY 

and  by  interbreeding  the  individuals  that  showed  these  traits  the 
ancon  race  of  sheep  was  established  (260).  The  most  famous 
discontinuous  variations  known  in  the  plant  kingdom  are  the 
distinct  species  of  evening  primroses,  which  have  originated  in 
recent  years  from  the  common  primrose,  and  have  been  so  well 
described  by  the  Dutch  naturalist  de  Vries  (272). 

(4)  Adaptations  of  Animals 

The  adaptations  of  animals  to  their  environment  is  so  usual  as 
to  pass  unnoticed.  Each  of  the  types  we  have  studied  is  fitted 
for  life  in  its  particular  habitat,  and  in  most  cases  in  no  other,  a 
fact  which,  as  we  have  seen,  accounts  largely  for  the  geographical 
distribution  of  organisms.  Animals  not  only  are  adjusted  to  their 
surroundings,  but  possess  structures  which  serve  them  in  obtain- 
ing food,  in  defending  themselves  and  their  young,  and  in  main- 
taining supremacy  among  individuals  of  their  kind.  The  cray- 
fish is  adapted  to  its  environment  by  the  possession  of  gills  for 
breathing,  eyes  for  detecting  moving  objects  in  the  water,  a  hard 
exoskeleton  to  protect  it  from  various  enemies,  pinchers  for  offense 
and  defense,  and  a  positive  thigmotropic  reaction  which  causes 
it  to  seek  a  dark  spot  out  of  range  of  marauders. 

Insects  are  especially  favorable  for  illustrating  adaptations. 
The  following  structures  of  the  honeybee  are  excellent  examples 
of  adaptations:  the  hairs  and  legs  for  gathering  pollen,  the  ali- 
mentary canal  for  obtaining  and  storing  nectar,  the  mandibles 
for  molding  wax,  the  sting  for  self-defense,  for  the  defense  of  the 
hive,  and,  in  the  case  of  the  queen,  for  use  in  determining  her 
supremacy.  The  origin  of  adaptations  is  a  moot  question  among 
biologists.  They  are  variations  which  have  been  inherited,  and 
have  become  more  pronounced  in  the  course  of  countless  genera- 
tions. 


GENERAL  ZOOLOGICAL  FACTS   AND  THEORIES        283 


(5)  The  Struggle  for  Existence 

It  has  been  estimated  that  if  it  were  possible  for  oysters 
to  breed  unmolested  by  their  enemies,  the  ocean  would  be  a 
solid  mass  of  bivalves  in  four  years.  Again,  if  a  pair  of  com- 
mon robins,  which  rear  on  an  average  four  young  per  year, 
were  allowed  to  live  and  produce  young  year  after  year,  and  if 
all  of  the  offspring  were  also  allowed  to  live  and  produce  young, 
the  descendants  of  the  first  pair  would  total  over  one  hundred 
thousand  at  the  end  of  the  tenth  year,  and  over  twenty  billion 
at  the  end  of  the  twentieth  year,  as  shown  in  Table  XVI  (256). 

TABLE  XVI 

THE.  YEARLY  INCREASE   IN   THE    NUMBER   OF   ROBINS   IF    ALLOWED  TO   BREED 

UNMOLESTED 


NUMBER  OF  YEAR 

NUMBER  OF  ADULTS 
AT  BEGINNING  OF  YEAR 

NUMBER  OF  BREED- 
ING PAIRS  OF  ADULTS 

NUMBER  OF  YOUNG 
REARED 

I 

2 

I 

4 

2 

6 

3 

12 

3 

18 

9 

36 

4 

54 

27 

108 

5 

162 

81 

324 

6 

486 

243 

972 

7 

1458 

729 

2916 

8 

4374 

2187 

8748 

9 

13122 

6561 

26244 

10 

39366 

19683 

78732 

Number  of  robins  at  end  of  loth  year 
Number  of  robins  at  end  of  20th  year 


118,098 
20,913,948,846 


What  really  happens,  however,  is  that  a  few  of  the  birds  may 
succeed  in  establishing  themselves  in  neighboring  regions,  but 
most  of  the  others  perish.  The  final  result  is  a  state  of  practical 
equilibrium  between  the  number  of  robins  and  the  factors  that 


284  AN  INTRODUCTION  TO  ZOOLOGY 

influence  their  life  activities,  so  that  we  are  able  to  see  very  little 
if  any  increase,  but  even  a  possible  decrease,  in  the  number  of 
birds  that  greet  us  with  the  coming  of  each  spring. 

These  animals  are  not  peculiar  in  any  way,  since  any  species 
that  might  be  selected  would  soon  cover  the  earth  with  its  de- 
scendants if  allowed  to  reproduce  unchecked.  In  most  cases  the 
number  of  individuals  of  a  species  is  practically  constant  from 
year  to  year.  This  is  the  result  of  the  struggle  for  existence. 
An  animal  must  contend  with  its  surroundings;  Euglena  must 
have  light  to  carry  on  its  life  processes;  and  the  honeybee  must 
store  up  provisions  and  make  its  hive  secure  in  order  to  survive 
the  winter. 

The  habitat  of  a  species  is  never  large  enough  to  accommodate 
all  the  individuals  of  a  species,  and  an  animal  is  forced  to  battle 
for  life  against  others  of  its  kind.  The  stronger,  other  things 
being  equal,  always  wins  in  the  struggle,  in  other  words,  the  fittest 
survives.  For  example,  in  poor  seasons  for  gathering  honey, 
bees  from  one  hive  attack  those  of  another,  and,  in  many  cases, 
carry  off  the  pollen  and  honey,  leaving  their  victims  to  starve. 
Other  battles  occur  between  individuals  of  different  species. 
Whatever  the  cause  of  the  struggle,  it  is  always  the  weakest  that 
is  vanquished.  The  strongest  individuals  of  a  species  live  on 
and  keep  up  the  vigor  of  the  race,  this  being  one  of  the  advan- 
tages of  the  elimination  of  the  unfit. 

(6)  Heredity 

Heredity  has  been  defined  (p.  79)  as  the  "  resemblance  of  child 
to  parent."  "  A  character  may  be  said  to  be  inherited  when  it 
always,  in  one  generation  after  another,  is  one  of  the  characters  of 
the  species,  ..."  (269,  p.  15).  The  characters  of  an  organism 
depend  on  the  nature  of  its  surroundings,  especially  during  de- 
velopment, and  on  the  constitution  of  the  part  of  the  parent  from 
which  it  grew.  In  Metazoons  this  may  be  a  bud,  as  in  Hydra, 
or  an  unfertilized  egg  cell,  as  in  the  drone  bee,  but  is  usually  an 
egg  fertilized  by  a  sperm. 


GENERAL  ZOOLOGICAL  FACTS  AND  THEORIES        285 

We  may  study  inheritance  by  examining  the  germ  cells,  by 
compiling  statistics,  or  by  the  experimental  method  of  cross- 
breeding and  subsequent  analysis  of  the  characters  of  the  off- 
spring. It  is  not  possible  in  any  case  to  predict  what  characters 
will  be  found  in  the  offspring  of  known  parents,  although  in 
certain  instances,  to  be  brought  out  later,  this  may  be  done  with 
some  degree  of  accuracy. 

(7)  The  Effects  of  Isolation 

Most  evolutionists  believe  that  a  necessary  condition  for  the 
formation  of  new  species  is  the  isolation,  separation,  or  segre- 
gation of  certain  individuals  from  others  of  their  kind.  This  may 
be  due  to  either  geographical  or  physiological  causes. 

GEOGRAPHICAL  ISOLATION.  —  "  In  the  case  of  geographic  or  topo- 
graphic isolation  the  isolated  group  or  groups  of  individuals  are 
actually  in  another  region  or  locality  from  the  rest  of  the  species, 
this  being  the  result  of  migration,  voluntary  or  involuntary  " 
(250,  p.  234).  We  have  shown  (p.  284)  that  normally  there  is 
an  overproduction  of  animals,  and  that  dispersal  is  necessary  for 
the  preservation  of  life.  "  In  regions  broken  by  few  barriers, 
migration  and  interbreeding  being  allowed,  we  find  widely 
distributed  species,  homogeneous  in  their  character,  the  mem- 
bers showing  individual  fluctuation  and  climatic  effects,  but 
remaining  uniform  in  most  regards.  ...  In  regions  broken  by 
barriers  which  isolate  groups  of  individuals  we  find  a  great 
number  of  related  species,  though  in  most  cases  the  same  region 
contains  a  smaller  number  of  genera  or  families.  .  .  .  Given 
any  species  in  any  region,  the  nearest  related  species  is  not  likely 
to  be  found  in  the  same  region  nor  in  a  remote  region,  but  in  a 
neighboring  district  separated  from  the  first  by  a  barrier  of  some 
sort"  (248). 

The  fauna  of  islands  show  in  a  striking  manner  the  effects  of 
isolation.  Some  of  these  i?lands,  if  not  of  volcanic  origin  and 
their  animals  the  descendants  of  individuals  that  were  driven 


286  AN  INTRODUCTION   TO  ZOOLOGY 

there  by  winds  or  carried  on  floating  objects,  have  for  a  very 
long  time  been  isolated,  and  this  isolation  has  so  changed  the 
character  of  these  animals  that  they  are  now  recognized  as 
distinct  species.  There  can  be  no  doubt  that  the  species  origi- 
nated from  individuals  from  the  mainland,  and  that  the  most 
important  factor  in  their  transmutation  has  been  geographical 
isolation  (274). 

PHYSIOLOGICAL  ISOLATION.  —  Internal  differences  may  cause 
physiological  isolation,  a  factor  in  the  origin  of  species  as  impor- 
tant probably  as  geographical  isolation.  Physiological,  or,  as  it  is 
sometimes  called,  sexual  isolation  is  caused  by  "  the  influence 
of  some  variation  tending  to  make  difficult  or  impossible  wholly 
free  and  miscellaneous  mating  or  breeding  inside  of  a  species. 
This  variation  may  be  of  purely  physiological  character  or  may 
be  a  structural  one:  that  is,  the  hindrance  to  mating  may  be  one 
of  instinctive  feeling,  a  '  race-feeling  '  depending  on  an  antipathy 
to  odour,  to  age,  to  appearance,  etc.,  or  may  be  a  slight  modifica- 
tion of  the  copulatory  organs  making  such  mating  difficult,  or 
even  a  modification  of  the  egg  or  the  spermatozoids  making  fer- 
tilization difficult.  It  is  a  well-known  fact  thai:  numerous 
varieties  of  domesticated  animal  speHes  rarely  breed  together, 
although  quite  able  to,  and  provided  with,  full  opportunity.  On 
the  other  hand,  animals  of  different  species  which  in  Nature 
rarely  or  never  breed  together  may,  if  kept  long  in  confinement, 
as  in  zoological  gardens,  mate  and  produce  young.  In  each  case 
there  seems  to  be  question  of  a  '  race-feeling ' ;  in  the  first  case 
a  sexual  aversion  keeping  apart  individuals  of  the  same  species, 
in  the  second  the  breaking  down  of  race-feeling  that  in  Nature 
has  sufficed  to  prevent  hybridizing  "  (250,  p.  245).  In  many 
cases,  if  interbreeding  does  occur,  the  hybrid  offspring  are  sterile, 
e.g.  the  mule  produced  by  a  cross  between  a  female  horse  and  a 
male  ass.  By  physiological  isolation,  then,  groups  of  individuals 
are  formed  within  the  same  region.  The  result  is  in  every  way 
similar  to  segregation  by  geographical  barriers. 


GENERAL  ZOOLOGICAL   FACTS   AND  THEORIES        287 

b.  Theories 
(i)  Heredity 

THE  INHERITANCE  OF  ACQUIRED  CHARACTERS. — Up  to  the 
middle  of  the  nineteenth  century  biologists  quite  generally  believed 
that  acquired  characters  are  inherited.  Lamarck  in  his  "  Fourth 
Law  of  Evolution  "  says:  "  All  that  has  been  acquired,  begun  or 
changed  in  the  structure  of  individuals  in  their  lifetime,  is  pre- 
served in  reproduction  and  transmitted  to  the  new  individuals 
which  spring  from  those  which  have  inherited  the  change." 
Darwin  accounts  for  many  adaptations  by  the  inheritance  of 
acquired  characters.  Many  controversies  arise  because  of  mis- 
understandings as  to  what  really  constitutes  an  acquired  charac- 
ter. Parts  of  a  man's  body  may  become  changed  by  use  or  dis- 
use, e.g.  the  arm  of  a  blacksmith  reaches  a  size  above  the  normal 
because  of  constant  use.  If  this  characteristic  influences  the 
germ  cells  of  the  blacksmith,  and  reappears  in  his  offspring,  we 
have  an  illustration  of  the  transmission  of  an  acquired  character. 

Weismann  (278)  is  the  foremost  opponent  of  the  belief  in  this 
theory.  He  led  scientists  to  examine  critically  all  reported  cases, 
and  as  a  result  it  was  found  that  no  case  really  shows  that  a  charac- 
ter was  not  inborn  instead  of  acquired.  Mutilations,  such  as  the 
severing  of  the  tails  of  sheep,  have  been  practiced  for  countless 
generations  without  affecting  the  tailed  condition  of  each  succeed- 
ing generation.  Many  supposed  cases  of  the  inheritance  of 
characters  produced  in  individuals  by  climatic  or  other  external 
factors  are  nothing  more  than  the  effects  of  the  external  stimuli 
upon  the  developing  organism.  For  example,  certain  snails  if 
reared  in  small  vessels  of  water  develop  into  small  adults,  but 
if  the  eggs  of  these  dwarfs  are  allowed  to  develop  and  the  larvae 
to  grow  to  maturity  in  a  large  vessel  of  water,  the  normal  size 
is  regained. 

It  seems  probable  that  whenever  an  organism  is  changed  by 
its  environment  and  the  change  is  transmitted  to  its  offspring, 


288  AN  INTRODUCTION  TO  ZOOLOGY 

the  germ  cells  as  well  as  the  body  cells  are  affected  by  the  exter- 
nal stimuli,  and  the  body  cells  have  no  effect  upon  the  germ  cells. 
The  theory  is  also  apparently  valueless  in  explaining  inheritance 
in  the  Protozoa  (p.  79). 

THE  CONTINUITY  OF  THE  GERM  PLASM. — The  present  wide- 
spread belief  in  the  theory  of  the  continuity  of  the  germ  plasm  is 
largely  due  to  the  efforts  of  Weismann.  We  have  already  illus- 
trated the  theory  in  a  general  way,  by  the  life  history  of  Volwx 
(p.  98,  Fig.  47),  and  in  another  place  (p.  32)  have  pointed  out 
that  the  chromosomes  are  thought  by  many  to  be  the  bearers  of 
hereditary  characters. 

According  to  Weismann,  there  are  "  two  great  categories  of 
living  substance  —  hereditary  substance  or  idioplasm,  and  '  nu- 
tritive substance  '  or  trophoplasm  "  (278,  Vol.  I,  p.  341).  He 
recognizes  the  chromatin  as  the  idioplasm,  and  calls  the  idioplasm 
of  the  germ  cells,  germ  plasm.  This  germ  plasm  is  "  never 
formed  de  now,  but  it  grows  and  increases  ceaselessly;  it  is  handed 
on  from  one  generation  to  another  like  a  long  root  creeping 
through  the  earth,  from  which  at  regular  distances  shoots  grow  up 
and  become  plants,  the  individuals  of  the  successive  generations  " 
(278,  Vol.  I,  p.  416). 

The  details  of  Weismann's  theory,  and  the  ingenious  explana- 
tions of  such  phenomena  as  cellular  differentiation  and  regenera- 
tion, are  extremely  complicated.  Briefly  stated,  the  mechanism 
is  conceived  by  Weismann  as  follows:  Each  large  chromosome 
of  a  germ  cell  is  composed  of  chromatin  granules,  called  "  ids." 
Each  id  is  either  male  or  female,  and  contains  all  of  the  "  com- 
plexes of  primary  constituents  necessary  to  the  production  of  a 
complete  individual  "  (278,  p.  349).  The  production  of  a  par- 
ticular part  of  an  organism  is  the  role  of  the  primary  constituents 
of  the  id,  and,  since  these  determine  the  nature  of  the  part  pro- 
duced, they  are  named  "  determinants."  Finally  the  determi- 
nants are  composed  of  one  or  more  particles,  called  "  biophors." 
These  are  "  far  below  the  limits  of  visibility,"  but  are  "  larger 
than  any  chemical  molecules  because  they  themselves  consist  of 


GENERAL    ZOOLOGICAL  FACTS  AND  THEORIES        289 

a  group  of  molecules,  among  which  are  some  of  complex  compo^i- 
tion,  and  therefore  of  relatively  considerable  size  "  (278,  p.  369). 

GALTON'S  LAW  OF  ANCESTRAL  INHERITANCE. — Francis  Galton, 
from  a  study  of  pedigreed  dogs  and  the  genealogical  records  of  the 
British  Peerage,  came  to  the  conclusion  that  in  an  organism  of 
bisexual  parentage,  "  The  two  parents  between  them  contribute 
on  the  average  one  half  of  each  inherited  faculty,  each  of  them 
contributing  one  quarter  of  it.  The  four  grandparents  contribute 
between  them  one  quarter,  or  each  of  them  one  sixteenth;  and  so 
on,  the  sum  of  the  series  -J-  +  \  +  i  +  TO  +  •  •  •  being  equal  to 
i,  as  it  should  be."  It  has  been  demonstrated  that  this  law  is 
only  an  approximation  of  the  real  result  (241). 

MENDEL'SLAW  OF  INHERITANCE  (227). — The  results  of  Mendel's 
experiments  in  hybridizing  peas  and  other  plants  were  published 
by  him  in  1866,  but  did  not  receive  any  attention  from  biologists 
until  1900,  when  they  were  discovered  in  an  obscure  periodical 
by  several  botanists.  Since  the  latter  date,  Mendel's  law  has 
become  a  favorite  object  of  investigation  with  both  botanists  and 
zoologists,  until  now  its  complexities  are  so  great  as  to  make  the 
details  of  the  subject  unintelligible  except  to  persons  working  in 
this  particular  field. 

An  outline  of  one  of  Mendel's  experiments  will  serve  to  illus- 
trate his  law  (269).  The  varieties  of  the  edible  pea  were  se- 
lected by  him  for  hybridization.  When  a  tall  pea  was  crossed 
with  a  dwarf  pea,  all  of  the  offspring  were  tall.  "  Tallness," 
therefore,  is  said  to  be  a  dominant  character  (D)  and  "dwarf- 
ness  "  a  recessive  character  (R). 

When  the  hybrids  were  allowed  to  fertilize  themselves,  three 
fourths  of  their  offspring  were  tall  (D)  and  one  fourth  dwarf 
(R) ;  there  were  no  intermediate  forms.  When  these  dwarfs  were 
allowed  to  fertilize  themselves,  only  dwarf  offspring  resulted. 
When  the  tall  peas  were  allowed  to  fertilize  themselves,  one  third 
of  them  produced  tall  offspring,  which  when  inbred  produced 
only  tall  peas;  the  other  two  thirds  when  inbred  produced  three 
fourths  tall  (D)  and  one  fourth  dwarf  (R)  offspring.  The  result 


AN  INTRODUCTION  TO  ZOOLOGY 


may  be  expressed  diagrammatically  as  in  Figure  157  (269). 
Many  other  plants  and  numerous  animals  have  been  experimented 
with;  in  some  cases  Mendel's  law  has  been  confirmed;  in  other 
cases  the  inheritance  is  apparently  determined  in  a  different  way. 
To  account  for  the  inheritance  of  characters  in  the  Mendelian 
ratio  of  three  dominants  (D)  to  one  recessive  (R),  the  theory  of 
the  segregation  of  germ  cells  has  been  proposed.  This  is  that 
the  "  germ  cells  or  gametes  produced  by  cross-bred  organisms  may 
in  respect  of  given  characters  be  of  the  pure  parental  types,  and 
consequently  incapable  of  transmitting  the  opposite  character; 
that  when  such  pure  similar  gametes  of  opposite  sexes  are  united 
in  fertilization,  the  individuals  so  formed  and  their  posterity  are 
free  from  all  taint  of  the  cross;  that  there  may  be  in  short,  per- 
fect or  almost  perfect  discontinuity  between  these  germs  in  re- 
spect of  one  of  each  pair  of  opposite  characters  "  (Bateson). 


TALL  9.X  DWARF-f          OR  DWARF 

•*•   v  O 


TALL  (DWARF) 


3/4   TALL 

II 

1/4    DW 

TALL  PEAS  FALL  INTO 
2  GROUPS,    1/3  PURE 
'DOMINANTS,  AND   2/3 
DOMINANT  WITH               DW 
DWARFNESS  LATENT 

TALL  OFFSPRING 
WITH  DWARFNESS           DW 
LATENT  PRODUCE 
1/4   PURE  DWARFS, 
*  AND  3/4    TALL. 
THE  LATTER  WHEN 
INBRED  ACT  AS  THE 
3/4    TALL  ABOVE 

DW/ 
DW/ 

ARF 
ARF 

ARF 

HF 
RF 

ALL                                         2/3  TALL  (DWARF) 
I                    > 

I           I 

kLL                %  TALL                           '/4  DW, 

RF 
RF 

»LL     V3  TALL               2/3   TALL             DW^ 
.(DWARF) 

KLL          TALL 

PARENTS  PRODUCE  ALL 
TALL  DOMINANTS 
CONTAINING  DWARFNESS 
IN  A  LATENT  CONDITION 


HYBRID  OFFSPRING  WHEN 
INBRED  PRODUCE    &/4 
TALL    AND  1/4     DWARF 


DWARFS  WHEN  INBRED 
BRED  TRUE,    ALWAYS 
GIVING  RISE  TO  DWARFS 


FIG.  157.  Diagram  showing  the  inheritance  of  tallness  and  dwarf  ness  in  peas 
according  to  Mendel's  Law.     (Modified  after  Thomson.) 


GENERAL  ZOOLOGICAL  FACTS  AND   THEORIES        291 

(2)  Evolution 

INTRODUCTION.  —  In  the  list  of  phyla  of  the  animal  kingdom  on 
pages  5  to  6,  the  approximate  number  of  described  species  be- 
longing to  each  is  given.  The  sum  total  of  all  these  is  265,050 
of  living  and  39,905  of  fossil  forms.1  The  origin  of  these  different 
species  may  be  accounted  for  in  three  ways:  (i)  by  special  creation 
(see  p.  8),  (2)  by  spontaneous  generation  (see  p.  8),  or  (3)  by 
evolution.  The  first  two  theories  are  no  longer  considered  seri- 
ously by  biologists;  some  of  the  evidence  for  evolution  will  be 
found  in  the  following  pages.  We  are  not  concerned  here  with 
the  question  of  the  origin  of  life;  but  shall  try  to  show  that  spe- 
cies have  evolved  from  one  another  or  have  descended  from 
common  ancestors. 

THE  ARGUMENTS  FOR  EVOLUTION.  —  Evidence  may  be  de- 
rived from  the  study  of  comparative  anatomy,  embryology, 
paleontology,  and  geographical  distribution  to  prove  that  organic 
evolution  has  taken  place. 

(a)  Comparative     Anatomy. — Homologous     structures,     i.e. 
structures  that  are  anatomically  similar,  point  to  a  common  an- 
cestry for  the  species  possessing  them.     The  fore  limbs  of  the  bird, 
dog,  man,  and  bat  (Fig.   158)  possess  the  same  fundamental 
structure,  though  each  is  modified  for  particular  functions.     Their 
similarity  is  explained  by  the  fact  that  the  animals  bearing  them 
have  all  descended  from  a  common  ancestor;  their  dissimilarity 
by  the  fact  that  they  are  used  for  different  purposes.     Vestigial 
organs,  such  as  the  muscles  of  the  ear,  and  the  appendix  of  man, 
are  structures  which  are  of  no  use  to  him  now,  but  were  functional 
in  his  ancestors.     They  indicate  to  us  some  of  the  characteristics 
of  these  ancestors  (256). 

(b)  Embryology.  —  This  subject  may  be  dismissed  by  referring 
the  reader  to  the  discussion  of  the  law  of  biogenesis  on  page  228. 

1  This  is  a  very  conservative  estimate  and  is  undoubtedly  a  lesser  number 
than  have  really  been  described.  The  number  is  increasing  every  year,  since 
new  species  are  being  described  almost  daily. 


2Q2 


AN  INTRODUCTION  TO  ZOOLOGY 


(c)  Paleontology  —  The  study  of  fossil  animals  has  brought 
out  some  remarkably  convincing  evidence  of  organic  evolution. 


FIG.  158.  Skeletons  of  the  fore  limbs  of  various  vertebrates,  a,  wing  of 
bird;  b,  fore  leg  of  dog ;  c,  arm  of  man ;  d,  wing  of  bat.  (From  Met- 
calf.) 

Table  XV  on  page  280  shows  that  in  the  oldest  strata  of  the  earth's 
crust  containing  organic  remains,  the  Cambrian  and  Silurian, 
only  invertebrates  existed.  Toward  the  end  of  the  latter  period 
fishes  made  their  appearance,  then  amphibians,  reptiles,  mam- 
mals, and  finally  Man.  Each  of  these  groups  is  of  a  higher  order 
than  the  last,  and  the  species  in  each  group  become  more 
complex  as  the  present  era  is  approached,  a  fact  not  shown  in 
the  table. 

Even  more  convincing  are  the  facts  revealed  by  a  study  of  the 
ancestors  of  a  single  species,  such  as  the  common  horse,  Equus 
(Fig.  159).  The  horses  of  the  Eocene  period  are  represented  by 
Orohippus,  an  animal  possessing  four-toed  fore  feet  and  three-toed 
hind  feet.  This  form  was  replaced  by  Mesohippus  in  the  Lower 
Miocene;  then  Protohippus  appears  in  the  Lower  Pliocene,  Plio- 
hippus  in  the  Pliocene,  and  finally  the  Equus  of  recent  times.  In 
these  forms  there  has  been  a  gradual  degeneration  of  certain 


GENERAL  ZOOLOGICAL   FACTS   AND  THEORIES        293 


toes,  and  a  corresponding  increase  in  the 
size  of  one  toe,  until  an  animal,  Equus,  is 
evolved  with  one  toe  on  each  foot  and  the 
splintlike  remains  of  what  were  functional 
toes  in  its  Eocene  ancestor. 

(d)  Geographical  Distribution.  —  The 
facts  of  geographical  distribution  all  seem 
to  show  that  species  originate  in  some  par- 
ticular place,  from  which  they  disperse  as 
their  numbers  increase.  When  groups  are 
prevented  from  mingling  with  others  of 
their  kind  by  environmental  barriers  or 
physiological  conditions,  the  factor  of  isola- 
tion, combined  with  other  factors,  result  in 
the  evolution  of  new  geographical  races  and 
finally  new  species.  Accordingly,  when 
species  that  resemble  each  other  closely  are 
found  occupying  neighboring  regions,  but 
separated  by  barriers,  we  conclude  that 
they  have  evolved  from  a  common  ances- 
tor, and  have  diverged  until  their  differ- 
ences are  considered  of  specific  rank. 

THE  THEORY  OF  NATURAL  SELECTION.  - 
The  origin  of  species,  as  conceived  by 
Darwin,  is  largely  due  to  natural  selection. 
We  may  gain  a  clear  idea  of  what  is  meant 
by  natural  selection  if  we  examine  the  re- 
sults of  artificial  selection.  For  example, 
there  are  at  the  present  time  over  one  hun- 
dred and  fifty  different  domestic  varieties  of 
pigeons;  a  few  of  these  are  shown  in  Figure 

FIG.  159.  Diagrams  illustrating  the  gradual  changes 
in  foot  structure  in  fossil  and  recent  species  of 
the  horse  family,  a,  bones  of  the  fore  foot ; 
b,  bones  of  the  hind  foot.  (From  Metcalf  after 
Marsh.) 


294  AN  INTRODUCTION  TO  ZOOLOGY 

1 60.  They  differ  widely  from  one  another  in  structure  and  habits 
but  in  many  cases  may  be  traced  back  to  a  single  ancestor,  the 
blue-rock  pigeon  (Columba  livia),  still  in  existence  in  its  wild  state 
(Fig.  1 60,  i).  The  theory  is  that  these  pigeons  have  been  origi- 
nated by  the  preservation  for  breeding  purposes  of  birds  that 
showed  favorable  variations. 

Now  animals  vary  in  a  wild  state  as  well  as  under  the  care  of 
man,  and  whenever  an  individual  appears  with  a  variation  of 
value  in  the  struggle  for  existence,  this  particular  animal  becomes 
one  of  the  fittest  to  survive,  and,  therefore,  lives  to  transmit  its 
favorable  variation  to  its  offspring.  In  other  words,  it  is  selected 
by  nature  just  as  a  domesticated  animal  with  a  favorable  varia- 
tion is  selected  by  Man.  As  an  example,  we  may  take  the  robin. 
We  have  shown  (p.  283)  that  this  bird  rears  a  number  of  young 
each  year,  yet  the  number  of  robins  remains  practically  constant 
from  year  to  year.  The  majority  are  killed  in  some  way.  Those 
that  are  strongest  and  are  able  to  weather  the  storms,  or  are 
endowed  with  extreme  speed  so  as  to  escape  their  enemies,  are 
selected  by  Nature,  while  weaker  or  slower  individuals  are 
doomed  to  destruction. 

Animals  become  adapted  to  their  surroundings  through  the 
operation  of  this  process  of  natural  selection.  The  wonderfully 
adapted  structures  on  the  legs  of  the  honeybee  for  gathering 
pollen  may  have  evolved  in  this  way. 

Of  recent  years  many  biologists  have  questioned  the  adequacy 
of  natural  selection  to  explain  all  the  modifications  of  species. 
This  was  never  claimed  for  the  theory,  even  by  Darwin,  for  he 
says,  in  the  introduction  to  the  Origin  of  Species,  "  I  am  convinced 
that  natural  selection  has  been  the  main,  but  not  the  exclusive, 
means  of  modification  "  (236). 

THE  THEORY  OF  ORTHOGENESIS.  — Many  phenomena  of  the  ev- 
olution of  organisms  cannot  be  explained  by  natural  selection.  For 
example,  the  development  of  structures  of  apparently  no  advan- 
tage to  the  animal,  such  as  the  arrangement  of  the  veins  in  the 
wings  of  insects,  and  the  development  of  modifications  harmful 


FIG.  1 60.  Varieties  of  domestic  pigeons,  i,  wild  blue-rock  pigeon  ;  2,  hom- 
ing pigeon;  3,  common  mongrel  pigeon;  4,  archangel;  5,  tumbler; 
6,  bald-headed  tumbler ;  7,  barb  ;  8,  pouter ;  9,  Russian  trumpeter ; 
10,  fairy  swallow;  n,  black-winged  swallow;  12,  fantail ;  13,  carrier; 
14  and  15,  bluetts.  (From  Metcalf.) 


GENERAL  ZOOLOGICAL  FACTS  AND  THEORIES        295 

to  the  race  and  leading  to  extinction,  structures  such  as  paleon- 
tologists tell  us  have  appeared  many  times  during  the  history  of 
the  animal  kingdom,  and  illustrated  by  the  Irish  stag  whose 
antlers  became  so  enormous  as  to  make  it  unfit  for  existence. 
There  appears  to  be  a  tendency  for  modifications  to  follow  certain 
definite  paths,  and  for  certain  structures  to  progress  in  prede- 
termined directions  regardless  of  their  utility,  and  without  the 
aid  of  natural  selection.  This  evolution  in  a  definite  predeter- 
mined direction  is  known  as  orthogenesis. 

There  are  a  number  of  theories  of  orthogenesis.  The  one  most 
in  favor  is  that  which  ascribes  the  control  of  modifications  to  the 
direct  effects  of  physicochemical  factors  on  organisms.  Some  of 
the  arguments  for  orthogenesis  are  derived  from  the  following 
phenomena:  the  presence  of  similar  variations  in  species  belong- 
ing to  one  family,  the  development  of  disadvantageous  structures 
leading  to  extinction  (e.g.  the  antlers  of  the  Irish  stag),  the  limits 
of  variation  due  to  the  chemical  composition  of  the  body,  the 
influence  of  one  organ  upon  another,  which  limits  variation,  and 
the  persistence  of  variations  in  definite  directions  so  that  evolu- 
tion is  not  in  radiating  lines  as  natural  selection  demands  (264). 
According  to  orthogenesis,  certain  lines  of  development  remain 
stationary,  while  others  advance. 

The  theory  of  orthogenesis  in  some  form  or  other  is  accepted 
by  many  specialists,  working  in  widely  separated  fields.  Of  these 
may  be  mentioned  Eimer  (239,  butterflies,  birds,  and  lizards), 
Whitman  (280,  pigeons),  Tower  (270,  beetles),  and  Ruthven  (266, 
snakes).  The  papers  of  these  men  furnish  excellent  concrete 
examples  of  how  orthogenesis  is  supposed  to  operate. 

THE  MUTATION  THEORY.  —  A  mutant  is  an  organism  that  dif- 
fers from  its  parent  in  certain  well-marked  characteristics,  which  it 
transmits  to  its  offspring.  De  Vries  is  the  foremost  exponent  of 
the  mutation  theory,  and  it  is  his  work  that  has  given  it  world- wide 
recognition.  The  conclusions  of  de  Vries  are  based  on  a  study  of 
the  evening  primrose  ((Enotkera  lamarckiana) .  The  mutation 
theory  may  be  stated  in  his  words  as  follows:  "  The  way  in  which 


296 


AN  INTRODUCTION  TO  ZOOLOGY 


one  species  originates  from  another  has  not  been  adequately 
explained.  The  current  belief  assumes  that  species  are  slowly 
changed  into  new  types.  In  contradiction  to  this  conception  the 
theory  of  mutation  assumes  that  new  species  and  varieties  are 
produced  from  existing  forms  by  sudden  leaps.  These  may  arise 
simultaneously  and  in  groups,  or  separately  at  more  or  less  widely 
distributed  periods  "  (272). 

While  directly  opposed  to  Darwin's  theory  of  the  origin  of  spe- 
cies in  regard  to  the  kinds  of  variations  selected,  de  Vries  gives 
Darwin  credit  for  the  discovery  of  the  "  great  principle  which 
rules  the  evolution  of  organisms.  It  is  the  principle  of  natural 
selection.  It  is  the  sifting  out  of  all  organisms  of  minor  worth 
through  the  struggle  for  life.  It  is  only  a  sieve,  and  not  a  force 
of  nature,  no  direct  cause  of  improvement,  as  many  of  Dar- 
win's adversaries,  and  unfortunately  many  of  his  followers  also, 
have  so  often  asserted.  It  is  only  a  sieve,  which  decides  which 
is  to  live,  and  what  is  to  die.  .  .  .  By  this  means  natural 
selection  is  the  one  directing  cause  of  the  broad  lines  of  evolution  " 
(272,  p.  6).  De  Vries  claims,  however,  that  "  species  and  varie- 
ties have  originated  by  mutation,  and  are,  at  present,  not  known 
to  originate  in  another  way." 

Two  sorts  of  species  are  recognized  by  de  Vries,  the  system- 
atic species  of  the  systematist,  and  elementary  species,  which  are 
more  numerous  and  are  described  as  "  any  form  which  remains 
constant  and  distinct  from  its  allies  "  (272,  p.  10).  Elementary 
species  have  unit  characters  which  are  indivisible  and  distinct 
in  inheritance.  All  organisms  are  not  mutable  to  the  same  degree, 
and  to  observe  the  origin  of  species,  "It  is  only  necessary  to 
have  a  plant  in  a  mutable  condition  "  (272,  p.  26). 

Since  the  publication  of  de  Vries'  results  biologists  have  exam- 
ined the  data  from  a  large  number  of  experiments,  but  only  a 
comparatively  small  amount  of  proof  is  available  to  support  the 
theory  from  the  zoological  side. 

Perhaps  the  best  attitude  to  take  toward  the  three  theories 
outlined  in  the  last  few  pages  is  that  of  Professor  Whitman,  who 


GENERAL  ZOOLOGICAL   FACTS   AND  THEORIES        297 

says,  "  natural  selection,  orthogenesis,  and  mutation  appear  to 
present  fundamental  contradictions;  but  I  believe  that  each  stands 
for  truth,  and  reconciliation  is  not  distant  "  (280). 

2.  THE  SOCIAL  LIFE  OF  ANIMALS 

Very  few  animals  lead  a  solitary  life,  and  even  those  that  may 
be  considered  solitary  are  influenced  indirectly  by  others,  just 
as  cats,  by  destroying  field  mice,  determine  the  number  of 
bumblebees  in  a  given  locality  (see  p.  262).  The  various  grades 
of  partnership  and  cooperation  are  recognized  by  the  terms 
commensalism,  symbiosis,  parasitism,  and  social  life. 

a.  Commensalism 

This  is  a  term  used  to  indicate  a  loose  association  of  two  kinds  of 
organisms  from  which  one  may  derive  benefit  at  the  expense  of  the 
other.  In  some  cases  one  species  eats  at  the  table  of  another 
without  contributing  anything  in  return.  The  guest  bumble- 
bee, that  frequents  the  nests  of  the  true  bumblebees  and  lives 
upon  the  food  stored  there,  belongs  to  this  class.  The  barnacles 
which  attach  themselves  to  the  skin  of  whales  derive  benefit 
from  being  carried  about,  and,  although  they  do  not  seriously 
inconvenience  their  bearer,  they  fail  to  pay  their  fare. 

b.  Symbiosis 

In  symbiosis  the  association  between  the  two  kinds  of  organisms 
is  intimate  and  persistent,  often  showing  marked  cooperation  and 
being  mutually  advantageous.  This  kind  of  partnership  may 
have  had  its  origin  in  the  more  simple  commensalism,  or  may  have 
arisen  from  parasitism.  Several  illustrations  of  symbiotic  rela- 
tions have  already  been  discussed  in  this  book,  that  between  two 
plants  (the  lichen,  p.  143),  that  between  a  plant  and  an  animal 
(Hydra  and  the  green  alga,  Zoochlorella,  p.  143),  and  that  between 
two  animals  (the  hermit  crab  and  a  Ccelenterate,  p.  143). 


298  AN  INTRODUCTION  TO  ZOOLOGY 

One  more  illustration  may  be  cited  to  show  how  remarkable 
the  relations  between  certain  plants  and  animals  may  become. 
The  flowers  of  the  genus  Yucca  depend  upon  a  single  species  of 
insect,  the  Pronuba  moth,  for  their  cross-pollination.  This  little 
white  moth  visits  the  flowers  in  the  evening.  It  scrapes  some 
pollen  from  a  stamen,  holds  it  underneath  its  head,  and  carries  it 
to  another  flower.  It  clings  to  the  pistil  of  this,  and,  thrustii 
its  ovipositor  through  the  wall  of  the  ovary,  lays  an  egg.  It  thei 
mounts  the  pistil,  and  forces  the  pollen  it  has  brought  down  int( 
the  stigmatic  tube.  Another  egg  is  laid  in  another  part  of 
ovary,  and  more  pollen  is  inserted  into  the  stigmatic  tube.  The 
processes  may  be  repeated  half  a  dozen  times  in  a  single  flower. 
The  advantage  to  the  flower  is,  of  course,  the  certainty  of  beii 
cross-pollinated  and  of  producing  seeds.  These  seeds  provide 
supply  of  food  for  the  larvae  that  hatch  from  the  eggs  laid  by  th( 
moth  in  the  ovary.  The  seeds  are  so  numerous  that  the  few  eaU 
by  the  larvae  may  well  be  spared  (267). 

c.  Parasitism 

A  parasitic  animal  is  one  that  lives  upon  another  organisi 
This  is  also  a  definition  of  a  predaceous  animal.  We  ma] 
distinguish  between  the  two,  however,  by  assuming  that  parasite 
are  always  carried  on  or  in  the  bodies  of  their  victims,  when 
predaceous  animals  are  free-living.  The  fleas  are  commoi 
examples  of  external  parasites.  The  malaria  parasite  (p. 
Fig.  42)  and  the  round  worm  (p.  160,  Fig.  81)  are  internal  para- 
sites. The  life  histories  of  these  species,  as  given  on  the 
referred  to,  will  suffice  to  illustrate  parasitic  habits,  but  certaii 
characteristics  resulting  from  this  kind  of  life  will  be  pointed  out 
briefly. 

Parasites  as  a  rule  are  comparatively  simple  in  structure, 
is  due  to  degeneration  and  not  to  a  low  position  in  the  anil 
kingdom.  In  many  cases  the  reproductive  organs  are  exceed- 
ingly well-developed,  whereas  organs  of  locomotion,  digestion, 
etc.,  become  almost  functionless. 


GENERAL  ZOOLOGICAL  FACTS  AND  THEORIES 


FIG.  161.  The  larval  stages  of  a  parasite,  Sacculin a  carcina.  A,  Nauplius; 
B,  Cypris;  C,  Cypris  attached  to  host ;  D,  E,  F,  stages  in  degeneration 
due  to  parasitic  habit.  (From  Sedgwick  after  Delage.) 

The  life  history  of  the  crustacean,  Sacculina  carcim,  a  parasite 
on  another  crustacean,  the  crab,  Car  emus  maenas,  may  be  con- 
sidered a  good  illustration  of  the  degeneration  of  parasites. 
Sacculina  hatches  as  a  Nauplius  (Fig.  161,  A),  which  soon  changes 
to  the  Cypris  stage  (B).  After  swimming  about  for  two  or  three 
days,  the  larva  attaches  itself  to  the  setae  of  a  crab  by  means  of  its 
antennae  (i).  The  base  of  the  seta  is  penetrated  by  the  hollow 


3oo 


AN  INTRODUCTION  TO  ZOOLOGY 


antenna,  and  the  larval  Sacculina  sheds  its  appendages,  shell, 
muscles,    and    excretory    organs.     What    remains    (E)    pass* 
through  the  antenna  into  the  body  of  the  crab  (F),  and  finally 
reaches  the  body  cavity.     A  system  of  roots  now  grows  out  in 
directions.     When  the  crab  molts,  the  saclike  body  of  the  para- 
site penetrates  to  the  outside,  resembling  a  tumor  consistii 
almost  entirely  of  reproductive  organs  (237). 


(d)  Social  Life 

That  there  is  a  continual  competition  among  individuals  am 
species  has  been  pointed  out  in  the  course  of  our  discussion  of 
geographical  distribution  (p.  275),  and  the  struggle  for  existem 
(p.    283).     However,   there  is  another  law  of  nature,   namely 
sociability,  not  so  far-reaching,  perhaps,  but  of  undoubted  im- 
portance to  the  welfare  of  many  and  diverse  species  of  animals 
Certain  of  the  social  species  have  already  been  mentioned  (\ 
264),  and  the  activities  of  the  honeybee,  an  insect  with  a  vei 
complex  social  life,  have  been  described  in  some  detail  (p.  233). 

The  beavers,  Rocky  Mountain  sheep,  and  prairie  dogs  (mam- 
mals), the  swallows,  herons,  and  sea  gulls  (birds),  and  the  ants, 
wasps,  and  termites  (insects)  are  famous  for  their  community 
life. 

The  advantages  of  living  together  are  many,  and  the  lives 
most  of  the  gregarious  animals  depend  upon  their  instinct  t( 
form  bands  and  colonies.     Darwin  recognized  the  value  of  sock 
life,  although  his  theories  were  based  largely  upon  the  effects  oi 
the  struggle  for  existence.     He  says  that  "  the  individuals  whicl 
took  the  greatest  pleasure  in  society  would  best  escape  various 
dangers;    while  those  that  cared  least  for  their  comrades,  an< 
lived  solitary,   would  perish  in   greater  numbers."     Deer,   fc 
example,  when  banded  together  may  escape  from  their  enemh 
whereas   single   individuals   would   be   destroyed.     The   Rocki 
Mountain  sheep  post  sentinels,  wh'ch  warn  the  main  herd  of 
approach  of  danger,  and  enable  all  to  seek  places  of  safety. 


GENERAL  ZOOLOGICAL  FACTS   AND  THEORIES       301 

The  gregarious  habit  is  also  of  value  for  offense,  as  in  the  case 
of  wolves  that  hunt  in  packs,  and  not  infrequently  the  weak  band 
together  and  attack  the  strong,  for  example,  the  swallows  that 
unite  to  drive  away  birds  of  prey.  Combined  activities  of  other 
kinds  also  occur;  certain  beetles  help  one  another  to  bury  small 
dead  animals  in  which  their  eggs  are  laid ;  the  monarch  butterflies, 
as  well  as  most  of  the  birds,  congregate  in  the  autumn,  and  migrate 
southward  together. 

The  steps  in  the  progress  toward  a  social  life  are  extremely 
interesting  (268). 

(1)  In  the  lowest  animals,  the  Protozoa,  there  are  certain  spe- 
cies, like  Volvox  (Fig.  46,  p.  97),  that  consist  of  colonies  of  cells 

ambling  the  early  stages  in  the  development  of  a  Metazoon 
(see  p.  107). 

(2)  Some  of  the  many-celled  animals  of  almost  every  phylum 
form  colonies,   which  contain    individuals    physically  different 

tuse  of  their  functions,  but  all  referable  to  a  common  type 
id  all  mutually  dependent;  for  example,  the  Portuguese  man- 
)f-war  among  the  Ccelenterates  (Fig.  67,  p.  142). 

(3)  The  love  of  mates  constitutes  a  third  step  in  social  prog- 
Many  of  the  higher  Metazoa,  such  as  insects  and  fish, 

)ut  especially  birds  and  mammals,  mate  and  become  mutually 
lelpful. 

(4)  The  love  of  mates  leads  to  family  life,  as  exemplified  by 
its,  bees,  birds,  beavers,  and  many  other  animals. 

(5)  And  finally  society,  which  is  the  combination  of  families, 
^presents  the  most  advanced  stage  of  social  life. 

In  conclusion  we  may  say  with    Kropotkin,    "  While  fully 
Imitting  that  force,  swiftness,  protective  colors,  cunning,  and 
idurance  of  hunger  and  cold,  which  are  mentioned  by  Darwin 
id  Wallace  as  so  many  qualities  making  the  individuals  or  the 
>ecies  the  fittest  under  certain  circumstances,  we  maintain  that 
inder  any  circumstances  sociability  is  the  greatest  advantage 
the  struggle  for  life.  .  .  .    The  fittest  are  thus  the  most  socia- 
animals,  and  sociability  appears  as  the  chief  factor  of  evo- 


302  AN  INTRODUCTION  TO  ZOOLOGY 

lution,  both  directly,  by  securing  the  well-being  of  the  species 
while  diminishing  the  waste  of  energy,  and  indirectly  by  favoring 
the  growth  of  intelligence.  .  .  .  Therefore  combine  —  practice 
mutual  aid.  That  is  the  surest  means  of  giving  to  each  and  to 
all  the  greatest  safety,  the  best  guarantee  of  existence  and  prog- 
ress—  bodily,  intellectual,  and  moral.  That  is  what  natui 
teaches  us." 

3.  REFLEXES,  INSTINCTS,  AND  THE  ANIMAL  MIND 
a.  Reflexes 

In  the  Metazoa  the  reflex,  carried  on  either  consciously  or  ui 
consciously,  is  considered  the  physiological  unit  of  nervous  ac- 
tivity.    It  requires  in  its  simplest  form  a  sensory  neuron,  th( 
receptor,  a  motor  neuron,  the  adjuster,  and  the  organ  stimulal 
to  activity,  the  effector  (see  p.  179).     The  stimulus  passes  froi 
the  receptor  to  the  adjuster  and  is  reflected  to  the  effector.     Tl 
apparent  reflection  suggested  the  term  "  reflex." 

The  reflex,  according  to  the  above  idea,  is  operative  only  in  tl 
case  of  animals  with  nervous  systems.     If  we  would  include 
certain  lower  Metazoa  and  the  Protozoa,  our  definition  must 
changed  so  as  to  take  into  account  the  action,  and  not  tl 
mechanism  that  is  responsible  for  the  action.     According  to  this 
view,  any  simple  response  to  a  stimulus  is  a  reflex.    The  avoiding 
reaction  of  Paramecium  (Fig.  33,  p.  74)  is  known  as  a  reflex,  an< 
the  activities  of  Paramecia  and  many  other  lower  organisms  ai 
supposed  by  some  to  be  a  series  of  reflexes  to  external  stimuli. 
But  if  examined  carefully,  the  reactions  of  lower  organisms  ai 
not  as  simple  as  they  at  first  appear.     For  example,  the  avoidii 
reaction  of  Paramecium  is  not  always  exactly  the  same, 
the  animal  be  taken  as  a  center  about  which  a  sphere  is  described, 
with  a  radius  several  times  the  length  of  the  body,  then  as  a  re 
suit  of  the  avoiding  reaction  the  animal  may  traverse  the  periph- 
eral surface  of  this  sphere  at  any  point,  moving  at  the  time  either 
backward  or  forward.     Paramecium,  in  spite  of  its  curious  limi- 


GENERAL  ZOOLOGICAL  FACTS  AND  THEORIES       303 

tations  as  to  method  of  movement,  is  as  free  to  vary  its  relations 
to  the  environment  in  response  to  a  stimulus  as  an  organism  of  its 
form  and  structure  could  conceivably  be.  Such  behavior  does 
not  fall  within  the  concept  of  a  reflex  if  the  latter  is  defined  as  a 
uniform  reaction  "  (246,  p.  279).  Jennings  has  pointed  out  that 
before  the  reflex  can  be  considered  a  simple  invariable  unit  for 
behavior,  the  physiological  condition  of  the  organism  must  be 
taken  into  account.  Such  a  unit  might  be  illustrated  as  follows: 
stimulus  A,  acting  upon  an  animal  whose  physiological  state  is 
B,  gives  reaction  C. 

b.  Instinct 

Instinct  is  "  the  faculty  of  acting  in  such  a  way  as  to  produce 
irtain  ends,  without  foresight  of  the  ends,  and  without  previous 
lucation  in  the  performance  "  (James).     Instinctive  acts  are 
complex.     They  are  performed  in  a  similar  manner  by  all 
lembers  of  the  same  sex  and  race.     Those  of  the  social  animals 
remarkably  varied.     Our  study  of  the  honeybee  (Chap.  XII) 
given  us  sufficient  knowledge  of  this  insect  to  warrant  our 
sing  it  to  illustrate  certain  kinds  of  instincts. 
The  queen's  nuptial  flight  and  her  copulation  with  the  drone  is 
istinctive.     She  acts  without  previous  experience,  and  accom- 
lishes  the  filling  of  her  spermatheca  with  spermatozoa  without 
apparently  realizing  the  purpose  of  the  whole  performance.   The 
selection  of  a  hollow  tree  by  worker  scouts,  and  the  cleaning  of  it 
in  preparation  for  the  swarm,  is  instinctive.     The  many  activities 
that  follow  the  advent  into  a  new  home,  such  as  crevice  chinking, 
varnishing,  wax  forming,  comb  building,  egg  laying,  gathering 
and  storing  pollen  and  nectar,  ripening  honey,  guarding,  cleaning, 
and  ventilating  the  hive,  and  caring  for  the  queen  and  young 
bees,  are  the  results  of  instincts. 

The  instinct  of  caring  for  the  young  is  of  special  interest,  since 
those  animals  that  care  for  their  young  succeed  in  rearing  a  larger 
proportion  than  those  that  allow  their  offspring  to  shift  for  them- 


304  AN   INTRODUCTION  TO  ZOOLOGY 

selves.    The  division  of  labor  among  the  workers  is  due  to  in- 
stinct;   although  each  individual  is  equipped  with  the  many 
structural  features  that  so  wonderfully  adapt  it  to  its  mode  of 
life,  only  one  sort  of  work  is  undertaken  at  a  time,  and  this  in  all 
cases  is  the  thing  most  needed  by  the  hive.     For  example,  as 
soon  as  a  hollow  tree  is  selected  and  cleaned,  the  workers  proceed 
to  secrete  wax  and  build  comb;    when  eggs  are  laid,  pollen  is 
gathered  and  stored  with  which  to  feed  the  forthcoming  h 
during  the  pollen  and  nectar  gathering  season,  each  worker 
lects  either  the  one  or  the  other,  and  the  pollen  gathered  on  or 
trip  is  all  of  one  color,  having  been  collected  evidently  from  01 
kind  of  flower.     Altruistic  instincts  such  as  these  are  especiall; 
abundant  in  social  animals,  and  among  these  may  be  classed 
social  instinct  itself.     On  the  other  hand,  the  instincts  of  feeding 
of  self-defense,  of  battling  for  the  supremacy  of  the  hive,  and 
playing  before  the  hive  on  bright  days,  may  be  said  to  be  egoistic 
These  definitions  and  examples  of  instinctive  acts  are  nc 
accepted  by  all  scientists;  and  of  recent  years  there  has  been 
tendency  to  consider  them  as  tropic  responses  to  stimuli.     Thi 
one  authority  attributes  all  the  activities  of  ants  and  bees 
reflex  responses  to  chemical  and  other  stimuli  (229).     The  ei 
nent  physiologist  Loeb  says:    "  My  investigations  on  the  helk 
tropism  of  animals  led  me  to  analyze  in  a  few  cases  the  conditioi 
which  determine  the  apparently  accidental  direction  of  aninu 
movements  which,  according  to  traditional  notions,  are  call* 
voluntary  or  instinctive.     Wherever  I  have  thus  far  investigate 
the  cause  of  such  '  voluntary  '  or  '  instinctive  '  movements 
animals,  I  have  without  exception  discovered  such  circumstances 
at  work  as  are  known  in  inanimate  nature  as  determinate  move- 
ments.    By  the  help  of  these  causes  it  is  possible  to  control  the 
'  voluntary  '  movements  of  a  living  animal  just  as  securely  and 
unequivocally  as  the  engineer  has  been  able  to  control  the  move- 
ments in  inanimate  nature.     What  has  been  taken  for  the  effect 
of  '  will '  or  '  instinct '  is  in  reality  the  effect  of  light,  of  gravity, 
of  friction,  of  chemical  forces,  etc."  (252,  p.    107).     According 


GENERAL  ZOOLOGICAL  FACTS  AND  THEORIES       305 

to  this  author,  "  For  the  inheritance  of  instincts  it  is  only  neces- 
sary that  the  egg  contain  certain  substances  —  which  will  deter- 
mine the  different  tropisms  —  and  the  conditions  for  producing 
bilateral  symmetry  of  the  embryo  "  (251). 

Many  other  competent  authorities  maintain  that  "  we  cannot 
exclude  a  psychic  element  from  the  definition  of  '  instinct '  with- 
out ignoring  its  very  nature  and  taking  it  for  a  reflex  motion, 
..."  (277,  p.  o),  and  that  in  both  the  lower  and  higher  animals 
"  the  behavior  is  not  as  a  rule  on  the  tropism  plan  —  a  set,  forced 
method  of  reacting  to  each  particular  agent  —  but  takes  place  in  a 
much  more  flexible,  less  directly  machinelike  way,  by  the  method 
of  trial  and  error  ..."  although  "  tropic  action  doubtless 
occurs  .  .  ."  (245,  p.  252). 

c.  The  Animal  Mind 

Mind  is  "  The  individual's  conscious  process,  together  with  the 
lispositions  and  predispositions  which  condition  it.  It  is  thus 
the  individual's  consciousness,  with  its  capabilities;  its  capabili- 
ties including  all  faculties,  powers,  capacities,  aptitudes,  and  dis- 
dtions,  acquired  and  innate  "  (226,  Vol.  II,  p.  82).  We  may 
lope  to  determine,  therefore,  the  character  of  an  animal's  mind 
>y  attempting  to  investigate  its  conscious  behavior. 

We  should  understand  at  the  beginning  of  cur  discussion  that 

icre  is  no  general  agreement  with  regard  to  the  presence  of  mind 
animals.  One  school  of  investigators  believes  that  all  animals 
ire  conscious;  another,  that  only  those  that  show  certain  kinds 
)f  behavior  should  be  considered  conscious;  and  a  third  school 
lolds  that  there  is  no  evidence  of  mind,  and  that  comparative 

sychology  should  be  abandoned,  and  behavior  should  be  ex- 
>lained  in  physiological  terms.  Certain  members  of  these  schools 

ilieve  with  Forel  that  it  is  "  possible  to  demonstrate  the  existence 
of  memory,  associations  of  sensory  images,  perceptions,  atten- 
tion, habits,  simple  powers  of  inference  from  analogy,  the  utiliza- 
tion of  individual  experience,  and  hence  distinct,  though  feeble, 
x 


306  AN  INTRODUCTION  TO  ZOOLOGY 

plastic  individual  deliberations  or  adaptations  "  (240,  p.  36). 
Other  authorities  consider  the  power  to  learn  by  individual 
experience,  for  example,  the  habit-forming  in  the  crayfish  (p.  223), 
as  evidence  of  mind.  And  still  others  deny  the  presence  of  mind, 
attributing  every  act  to  chemicophysical  processes. 

It  certainly  is  true  that  no  one  knows  whether  animals  are 
conscious  or  not.  Our  evidence  is  all  gained  by  inferences  from 
behavior.  The  conditions  under  which  the  facts  of  behavior  ai 
obtained  are  of  great  importance.  Anecdotes  are  of  little  value, 
since  the  observer  is  not  scientifically  trained,  and,  wishing  to  tell 
a  good  story,  tends  to  exaggerate  the  apparent  intelligence  of 
animal.  The  modern  method  is  that  of  carrying  out  definite 
experiments,  the  habits  of  the  species  and  past  experiences  of  tl 
individuals  being  known.  The  conclusion  is  then  based  on 
least  complex  interpretation  that  will  account  for  the  facts  of 
behavior  observed.  We  can,  however,  only  interpret  behavioi 
on  the  analogy  of  human  experience,  the  tendency  being  to 
cribe  human  traits  to  animals. 

Learning  by  experience,  a  process  considered  by  many  indica- 
tive of  intelligence,  is  one  of  the  criteria  of  the  presence  of  mind; 
but  the  modifiability  of  the  behavior  must  be  rapid,  or  tim( 
enough  for  a  change  in  body  structure  may  elapse.  Structure 
is  a  distinct  help  in  inferring  mind,  since  animals  with  a  nerv- 
ous system  similar  to  that  of  Man  must  be  affected  by  external 
stimuli  in  a  similar  way. 

"  We  know  not  where  consciousness  begins  in  the  animal 
world.  We  know  where  it  surely  resides  —  in  ourselves; 
know  where  it  exists  beyond  a  reasonable  doubt  —  in  the 
animals  of  structure  resembling  ours  which  rapidly  adapt  them- 
selves to  the  lessons  of  experience.  Beyond  this  point,  for  all 
we  know,  it  may  exist  in  simpler  and  simpler  forms  until  we  reach 
the  very  lowest  of  living  beings  "  (276,  p.  36). 


BIBLIOGRAPHY 


A  few  of  the  books  and  articles  that  have  been  consulted  in  preparing 
this  work  will  be  found  in  the  following  list.  They  have  been  selected 
chiefly  with  reference  to  their  accessibility  and  value  to  students,  and 
are  arranged  alphabetically  under  the  headings  of  the  various  chapters. 
Their  numbers,  corresponding  to  numbers  in  parentheses  in  the  text, 
will  facilitate  the  finding  of  any  particular  source  of  information. 

CHAPTER  I 
INTRODUCTION 

1.  Aspects  of  the  Species  Question.  —  Papers  read  at  a  symposium  at 

Chicago,  Jan.  i,  1908.     1908.      Amer.  Nat.,  vol.  42,  pp.  218-281. 

2.  Darwin,  C.,  1861.  —  The  Origin  of  Species  by  means  of  Natural  Selec- 

tion.    New  York. 

3.  Harmer,  S.  F.,  and  A.  E.  Shipley.  —  Editors.     The  Cambridge  Nat- 

ural History.     10  vols.     London. 

4.  liertwig,  R.,    1902.  —  Manual  of  Zoology.     Translated    by    J.    S. 

Kingsley.     New  York. 

5.  Lankester,  E.  R.  —  Editor.     A  Treatise  on  Zoology.     London. 

6.  Parker,  G.  H.    1908.  —  Zoological  Progress.     Amer.  Nat.,  vol.  42, 

pp.  115-133- 

7.  Parker,  T.  J.,  and  W.  A.  Haswell,  1897.  —  Text-book  of  Zoology. 

2  vols.     London. 

8.  Thomson,  J.  A.,  1906.  —  Outlines  of  Zoology.     4th  ed.     London. 

9.  Williston,  S.  W.,  1908.  —  What  is  a  Species?     Amer.  Nat.,  vol.  42, 

pp.  184-194. 

10.  Zittel,  K.  A.  von,  1900-1902.  —  Text-book  of  Paleontology.      2  vols. 

Translated  by  C.  R.  Eastman.     London. 

CHAPTER  II 
PHENOMENA   OF  LIFE 

11.  Biitschli,  O.,  1894.  —  Investigations  on  Microscopic  Foams  and  on 

Protoplasm.     London. 

12.  Dahlgren,  U.,  and  W.  A.  Kepner,  1908.  —  Text-book  of  the  Prin- 

ciples of  Animal  Histology.     New  York. 
307 


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13.  Lee,  F.  S.,  1908.  —  Physiology.     Amer.  Nat.,  vol.  42,  pp.  394-41  / 

14.  Locy,  W.  A.,  1908.  —  Biology  and  its  Makers.     New  York. 

15.  Loeb,  J.,  1906.  —  The  Dynamics  of  Living  Matter.     New  York. 

16.  Sedgwick,   W.   T.,  and  E.   B.    Wilson,  1899.  —  General   Biology. 

2d  ed.     New  York. 

17.  Verworm,  M.,  1899.  —  General  Physiology.     Translated  by  F.  E. 

Lee.     London. 

18.  Wilson,  E.  B.,  1899.  —  On  Protoplasmic  Structure  in  the  Eggs  of 

Echinoderms  and  Some  Other  Animals,  Journ.  of  Morph.,  vol.  15, 
Suppl.,  pp.  1-28. 

19.  ,  1900.  —  The     Cell   in    Development    and    Inheritance.     2d 

ed.     New  York. 

20.  ,  1908.  —  Biology.     New  York. 

CHAPTER   III 
THE  CELL  AND   THE   CELL  THEORY 

21.  Dahlgren,  U.,  and  W.  A.  Kepner,  1908.  —  Text-book  of  the  Prin- 

ciples of  Animal  Histology.     New  York. 

22.  Farmer,   J.  B.,   1903.  —  The  Structure  of  Animal  and  Vegetable 

Cells.     In  Lankester's  Treatise  on  Zoology,  Part  I,  2d  fascicle. 
London. 

23.  Hertwig,  O.,   1895.     The  Cell.     Translated  by  M.  Campbell  and 

edited  by  H.  J.  Campbell.     London. 

24.  Locy,  W.  A.,  1908.  —  Biology  and  its  Makers.     New  York. 

25.  Wilson,  E.  B.,  1900.  —  The  Cell  in  Development  and  Inheritance. 

2d  ed.     New  York. 

CHAPTER   IV 

AMEBA 

26.  Bernstein,  J.,  1900.  —  Chemotropische  Bewegungen  eines  Queck- 

silbertropfens.     Arch.  f.  d.  ges.  Physiol.,  Bd.  80,  pp.  628-637. 

27.  Berthold,  G.,  1886.  —  Studien  iiber  Protoplasmamechanik.     Leip- 

zig. 

28.  Biitschli,  O.,  1894.  —  Investigations  on  Microscopic  Foams  and  on 

Protoplasm.     London. 

29.  Calkins,  G.  N.,  1901.  —  The  Protozoa.     New  York. 

30.  ,  1905.  —  Evidences  of  a  Sexual  cycle  in  the  Life-History  of 

Amceba  proteus.     Archiv  f.  Protist.,  vol.  5,  pp.  1-16. 

31.  ,  1907.  —  The  Fertilization  of    Ameba    proteus.     Biol.    Bull., 

vol.  13,  pp.  219-230. 


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3°9 


32.  ,  1909.  —  Protozoology.     Philad. 

33.  Davenport,    C.    B.,    1897.  —  Experimental    Morphology,    vol.    I. 

New  York. 

34.  Bellinger,  O.  P.,  1906.  —  Locomotion  of  Amoebae  and  Allied  Forms. 

Journ.  of  Exp.  Zool.,  vol.  3,  pp.  337-358. 

35.  Harrington  and  Learning,  1900.  —  The  Reaction  of  Amoeba  to  Light 

of  Different  Colors.     Amer.  Journ.  Physiol.,  vol.  3,  pp.  9-18. 

36.  Hofer,  B.,  1889.  —  Experimentelle  Untersuchungen  iiber  den  Ein- 

fluss  des  Kerns  auf  das  Protoplasma.     Jenaische  Zeitschrift,  Bd. 
17,  105-176. 

37.  Jennings,   H.    S.  —  Methods   of    Cultivating    Amoeba   and    Other 

Protozoa  for  Class  Use.     Journ.  Appl.  Micros,  and  Lab.  Methods, 

vol.  6,  p.  2406. 
38. ,  —  A   Method   of  Demonstrating   the   External   Discharge  of 

the  Contractile  Vacuole.     Zool.  Anz.,  Bd.  27,  pp.  656-658. 
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41. ,  1906.  —  Behavior  of  the  Lower  Organisms.     New  York. 

42.  Kuhne,  W.,    1864.  —  Untersuchungen  iiber  das  Protoplasma  und 

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44.  Rhumbler,  L.,    1898.  —  Physikalische   Analyse  von   Lebenserschei- 

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45.  Schaudinn,  F.,  1895.  —  Uber  die  Theilung  von  Amoeba  binucleata 

Gruber.     Sitz.  Ber.  Ges.  Nat.  Freunde  Berlin,  pp.  130-141. 

46.  Sheel,  C.,  1899.  —  Beitrage  zur  Fortpflanzung  der  Amoben.      Fests 

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47.  Schulze,  F.  E.,  1875.  —  Rhizopodenstudien.     Arch.  f.  Mikr.  Anat., 

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48.  Verworm,  M.,  1899.  —  General  Physiology.     Translated  by  F.  E. 

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CHAPTER   V 
PARAMECIUM 

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Anat.  u.  Physiol.,  Suppl.,  Pts.  1-2,  pp.  147-157. 

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66. ,  and  C.  Jamieson,  1902.  —  The  Movements  and  Reactions  of 

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67.  Jenson,  P.,  1893.  —  Ueber  den  Geotropismus  niederer  Organismen. 

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68.  Lyon,  E.  P.,  1905.  —  On  the  Theory  of  Geotropism  in  Paramoecium. 

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69.  Maier,  H.  N.,  1903.  —  Ueber  den  feineren  Bau  der  Wimperapparate 

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70.  Mast,  S.  O.,  1909.  —  The  Reactions  of  Didinium  nasutum  (Stein), 

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71.  Maupas,   E.,    1888.  —  Recherches   Experimentales   sur   las   Multi- 

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72.  Mitrophanow,  P.,  1905.  —  Etude  sur  la  structure,  le  developpement 

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73.  Schaefer,  E.,  1904.  —  Theories  of  Ciliary  Movement.     Anat.  Anz., 

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CHAPTER  VI 
OTHER  PROTOZOA 

76.  Blackmail,  F.  F.,  1900.  —  The  Primitive  Algae  and  the  Flagellata. 

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77.  Bourne,  G.  C.,  1909.  —  Comparative  Anatomy  of  Animals.,  vol.  i, 

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78.  Calkins,  G.  N.,  1909.  —  Protozoology.     Philad. 

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80.  Jennings,  H.  S.,  1906.  —  The  Behavior  of    Lower  Organisms.     New 

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81.  Kent,  W.  S.,  1881.  —  Manual  of  the  Infusoria.     London. 

82.  Keuten,  J.,  1895.  —  Die  Kerntheilung  von  Euglena  viridis.     Zeit. 

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83.  Lang,    A.,    1901.  —  Lehrbuch    der    Vergleichenden  Anatomic    der 

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85.  Oltmanns,  F.,  1904.  —  Morphologic   und  Biologic  der   Algen,  Bd. 

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86.  Weismann,  A.,    1904.  —  The   Evolution  Theory.     London.    Trans- 

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CHAPTER   VII 
INTRODUCTION  TO   THE   METAZOA 

87.  Boveri,  T.,  1892.  —  Die  Entstehung  des  Gegensatzes  zwischen  den 

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88.  Calkins,  G.  N.,  and  S.  Cull,  1907.  —  The  Conjugation  of  Paramae- 

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312 


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90.  Haecker,  V.,   1897.  —  Die   Keimbahn  von  Cyclops.     Arch.   Mikr. 

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91.  Hatschek,  B.,  1886.  —  Zur  Entwicklung  des  Amphioxis.     Berlin. 

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95.  Martin,  H.  N.,  1894.  —  The   Human   Body.    6th  ed.     New  York. 

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99.  Wilson,  E.  B.,  1900.  —  The  Cell  in  Development  and  Inheritance. 

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CHAPTER   VIII 
HYDRA  AND   CCELENTERATES  IN   GENERAL 

101.  Bourne,  G.  C.,  1909.  —  Comparative  Anatomy  of  Animals,  vol.  i, 

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102.  Brauer,  A.,  1891.  —  Ueber  die  Entwicklung  von  Hydra.     Zeit.  f. 

wiss.  Zool.,  Bd.  52,  pp.  169-216. 

103.  Cambridge  Natural  History,  1906,  vol.  i. 

104.  Downing,  E.  R.,  1902.  —  Ingestion  and  Digestion  in  Hydra,  Science. 

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106.  ,  1908.  —  The    Ovogenesis    of    Hydra    fusca.     A  preliminary 

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107.  Frischholz,  E.,  1909.  —  Zur  Biologic  von  Hydra.     Biol.  Centralbl. 

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108.  Glaser,  O.  C.,  and  C.  M.  Sparrow,   1909.  —  The  Physiology  of 

Nematocysts.     Journ.  Exp.  Zool.,  vol.  6,  pp.  361-382, 


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109.  Hadzi,  J.,  1909.  —  Ueber  das  Nervensystem  von  Hydra.     Arbeit. 

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114.  Parker,  T.  J.,  and  W.  A.  Haswell.,  1897.  —  Text-book  of  Zoology. 

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116.  Reese,   A.    M.,    1909.  —  Variations   in    the  Tentacles   of    Hydra. 

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117.  Shipley,  A.  E.,  and  E.  W.  MacBride,  1904.  —  Zoology.    Cambridge. 

118.  Tannreuther,  G.  W.,  1908. —  The  Development  of  Hydra.  Biol. 

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Hydra.  Zool.  Anz.,  Bd.  33,  pp.  798-805. 

122.  Trembley,   A.,    1744.  —  Memoires   pour   servir   a  1'Histoire   d'un 

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123.  Wagner,  G.,  1904.  —  On  Some  Movements  and  Reactions  of  Hydra. 

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124.  Wilson,  E.  B.,  1891.  —  The  Heliotropism  of  Hydra.   Amer.  Nat., 

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CHAPTER  IX 
SPONGES,  FLAT  WORMS,  AND  ROUND  WORMS 

25.  Arnold,  A.,  1909.  —  Intracellular  and  general  digestive  processes 
in  Planariae.  Quart.  Journ.  Micro.  Sc.,  vol.  54,  pp.  207-220. 

:26.  Bourne,  G.  C.,  1908.  —  Comparative  Anatomy  of  Animals,  vol.  2. 
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127.  Cambridge  Natural  History.  —  Vol.  i,  1906,  vol.  2,  1896.  Lon- 
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3*4 


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128.  Drew,  G.  A.,  1907.  —  Invertebrate  Zoology.     Philadelphia. 

129.  Morgan,  T.  H.,  1901.  —  Regeneration.     New  York. 

130.  Parker,  G.  H.,  1909.  —  The  Origin  of  the  Nervous  System  and  its 

Appropriation  of  Effectors.      Pop.  Sc.  Monthly,  vol.   75,  pp.  56- 
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131.  Parker,  T.  J.,  and  W.  A.  Haswell,  1897.  —  Text-book  of  Zoology, 

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132.  Pratt,  H.  S.,  1902.     Invertebrate  Zoology.     Boston. 

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134.  Treatise  on  Zoology,  ed.  by  E.  R.  Lankester.     Vol.  2,  1900,  vol.  4, 

1901.     London. 


CHAPTER  X 
THE  EARTHWORM  AND  ANNELIDS  IN  GENERAL 

135.  Adams,  G.  P.,  1903.  —  On  the  Negative  and  Positive  Phototropisi 

of  the  Earthworm,  Allolobophora  foetida,  as  Determined  by  Light 
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136.  Barker,  1907.  —  The  Neuron  Theory.     Review  of  Harvey  Lectures 

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137.  Bourne,  G.  C.,  1908.  —  Comparative  Anatomy  of  Animals,  vol.  2. 

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138.  Bugnion,  E,  and  N.  Popoff,  1905.  —  La  spermatogenese  du  Lombrk 

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140.  Depdolla,  P.,   1905.  —  Untersuchungen  iiber   die    Spermatogenes 

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141.  Foot,  K.,  1898. — The  Cocoons  and  Eggs  of  Allolobophora  f 02 tida. 

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142.  — ,  and  E.  C.  Strobell,  1902.  —  Further  Notes  on  the  Cocoons 
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143.  Harrington,  N.  S.,  1899.  —  The  Calciferous  Glands  of  the  Earthworm 

with  Appendix  on  the  Circulation.  Journ.  Morph.,  Supp.,  vol. 
15,  pp.  105-168. 

144.  Hesse,  R.,  1896.  —  Untersuchungen  iiber  die  Organe  der  Lichtemp- 

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419. 


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146.  Johnston,  J.  B.,  1903.  —  On  the  Blood  Vessels,  their  Valves,  and  the 

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147.  Langdon,  F.  E.,  1895.  —  The  Sense-Organs  of  Lumbricus  agricola 

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148.  Marshall,   A.   M.,   and  C.   H.   Hurst,  1899.  —  Practical  Zoology. 

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149.  Morgan,  T.  H.,  1901. — Regeneration.     New  York. 

150.  Nagel,  W.  A.,  1896.  —  Der  Lichtsinn  augenloser  Thiere.     Jena. 

151.  Parker,  G.  H.,  1909.  —  The  Origin  of  the  Nervous  System  and  its 

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the  Earthworm,    Allolobophora  foetida.     Am.    Journ.    Physiol., 

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153. ,  and  C.  R.  Metcalf,  1906.  —  The  Reactions  of  Earthworms  to 

Salts.     Am.  Journ.  Physiol.,  vol.  17,  pp.  55-74. 
154.  Rice,  W.  J.,  1902.  —  Studies  in  Earthworm  Chloragogue.     Biol. 

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156.  Washburn,  M.  F.,  1908.  —  The  Animal  Mind.     New  York. 
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Journ.  Morph.,  vol.  3,  pp.  387-462. 


CHAPTER   XI 
THE   CRAYFISH  AND  ARTHROPODS  IN  GENERAL 

158.  Andrews,    E.    A.,    1904.  —  Crayfish    Spermatozoa.     Anat.    Anz., 

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[60. ,  1906.  —  Egg-laying  of  Crayfish.     Am.  Nat.,  vol.  40,  pp.  343- 

356. 
[61. ,    1906.  —  Partial   Regeneration   of   the   Sperm-receptacle   in 

Crayfish.     Journ.  Exp.  Zool.,  vol.  3,  pp.  121-128. 


316  BIBLIOGRAPHY 

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163.  ,  1908.  —  The  Young  of  the  Crayfishes  Astacus  and  Cambarus, 

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164.  Bell,    J.    C.,    1906.  —  The  Reactions   of   the    Crayfish.     Harvard 

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165.  ,  1906.  —  The  Reactions  of  the  Crayfish  to  Chemical  Stimuli. 

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167.  Chidester,  F.  E.,  1908.  —  Notes  on  the  Daily  Life  and  Food 

Cambarus  bartonius  bartoni.     Am.  Nat.,  vol.  42,  pp.  710-716. 

1 68.  Dearborn,  G.,  1900.  —  The  Individual  Psycho-physiology  of  th( 

Crayfish.     Am.  Journ.  Physiol.,  vol.  3,  pp.  404-433. 

169.  Faxon,  W.,   1885.  —  A  Revision  of  the  Astacidae.     Mem.    Mus. 

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170.  Huxley,  T.  H.  —  The  Crayfish.     New  York. 

171.  Korschelt,  E.,  and  K.  Heider,  1899.  —  Embryology  of  Invertebrates. 

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172.  Kreidl,   A.,    1893.  —  Weitere   Beitriige   zur   Physiologic   des   Ohi 

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173.  Marshall,  A.  M.,  and  H.  H.  Hurst,   1899.  —  Practical  Zoology. 

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174.  Miller,  W.  S.,  1895.  —  The  Anatomy  of  the  Heart  of  Cambarus. 

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175.  Montgomery,  T.  H.,  1906.  —  The  Analysis  of  Racial  Descent  in 

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176.  Morgan,  T.  H.,  1901.  —  Regeneration.     New  York. 

177.  ,  1903.  —  Evolution  and  Adaptation.     New  York. 

178. ,  1904.  —  Germ  Layers  and  Regeneration.     Arch.  Ent.  mech. 

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179.  Ortmann,  A.  E.,  1907.  —  The  Crayfishes  of  the  State  of  Pennsyl- 

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180.  Parker,  G.  H.,  1895.  —  The  Retina  and  Optic  Ganglia  in  Decapods, 

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i-73- 

181.  Prentiss,    C.   W.,    1901.  —  The   Otocyst   of   Decapod    Crustacea. 

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182.  Reed,  M.,  1904.  —  The  Regeneration  of  the  First  Leg  of  the  Cray- 

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183.  Sedgwick,  A.,  1909.  —  Text-book  of  Zoology,  vol.  3.     London. 


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185.  Treatise  on  Zoology,  ed.  by  E.  R.  Lankester,  1909.  Vol.  7,  3d  fas- 

cicle.    London. 

186.  Washburn,  M.  F.,  1908.  —  The  Animal  Mind.     New  York. 

187.  Weismann,  A.,  1904.  —  The  Evolution  Theory,  vol.  2,  translated 

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188.  Yerkes,  R.  M.,  and  G.  E.  Huggins,  1903.  —  Habit  Formation  in 

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189.  Zeleny,  C.,  1905.  —  The  Relation  of  the  Degree  of  Injury  to  the 

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CHAPTER  XII 
THE  HONEYBEE  AND  BEES  IN  GENERAL 

tgi.  Benton,  F.,  1896.  —  The  Honey  Bee.  Bull,  i,  N.  S.  (Revised 
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[93.  Bonnier,  G.,  1901.  —  L'accoutumance  des  abeilles  et  la  couleur  des 
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Zool.  Anz.,  Bd.  29,  pp.  299-323. 
[96.  Buttel-Reepen,  H.  von,  1900.  —  Sind  die  Bienen  Reflexmaschine? 
Biol.   Cent.,  Bd.   20,  pp.  97-109;   130-144;   177-193;   209-224; 
289-304. 

[97.  Cambridge  Natural  History,  1899,  vol.  6.     London. 
[98.  Cheshire,  F.  R.,  1886.  —  Bees  and  Bee-Keeping.     2  vols.     London. 
[99.  Comstock,  J.  H.,  and  A.  B.  Comstock,  1907.  —  Manual  for  the 

Study  of  Insects,  7th  ed.     Ithaca. 

Cowan,  T.  W.,  1904.  —  The  Honey  Bee.     2d  ed.     London. 
>i.  Dickel,  F.,  1908.  —  Zur  Frage  nach  der  Geschlechtsbestimmung  der 
Honigbiene.     Zool.  Anz.,  Bd.  33,  pp.  222-236;  Bd.  34,  1909,  pp. 
212-223;  236-248. 
>2.  Dreyling,  L.,  1905.  —  Die  Wachsbereitenden  Organe  bei  den  gesellig 


BIBLIOGRAPHY 


lebenden  Bienen.     Zool.  Jahrb.,    Abth.  Morph.,  Bd.  22,  pp.  2! 

330- 

203.  Gibson,  W.  H.,  1898.  —  My  Studio  Neighbors.     New  York. 

204.  Graenicher,  S.,  1909.  —  Wisconsin  Flowers  and  their  Pollinatior 

Bull.  Wise.  Nat.  Hist.  Soc.,  vol.  7,  pp.  19-77. 

205.  Kathariner,  L.,   1903.  —  Versuche  iiber  die  Art  der  Orientierui 

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206.  Kellogg,    V.    L.,    1903.  —  Some  Insect   Reflexes.     Science,  N. 

vol.  18,  pp.  693-696. 
207. ,  1905.  —  American  Insects.     New  York. 

208.  Lovell,  J.  H.,  1909.  —  The  Color  Sense  of  the  Honey  Bee — 

conspicuousness  an  advantage  to  flowers?   Am.  Nat.,  vol.  43,  pj 
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209.  Mark,  E.  L.,  and  M.  Copeland,  1906.  —  Some  Stages  in  the  Sperm; 

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42,  pp.  103-112. 

210.  Meves,  F.,  1907.  —  Die  Spermatocytentheilungen  bei  der  Hone} 

biene  (Apis  mellifica  L),  nebst  Bemerkungen  iiber  Chromatini 
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211.  Morgan,   T.    H.,    1907.  —  The    Cause   of   Gynandromorphism 

Insects.     Am  Nat.,  vol.  41,  pp.  715-718. 
212. ,  1907.  —  Experimental  Zoology.     New  York. 

213.  Mueller,  H.,  1873.  —  Die  Befruchtung  der  Blumen  durch  Insektt 

214.  Packard,  A.  S.,  1903.  —  Text-Book  of  Entomology.     New  York. 

215.  Petrunkewitsch,  A.,  1901.  —  Die  Richtungskorper  und  ihr  Schicl 

sal  im  befruchteten  und  unbefruchteten  Bienenei.     Zool.  Jahrl 
Abth.  Morph.,  Bd.  14,  pp.  291-310. 

216.  Phillips,  E.  F.,  1905.  —  Structure  and  Development  of  the  Coi 

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217.  Plateau,  F.,  1907.  —  Les  insectes  et  la  couleur  des  fleurs.     L'Am 

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218.  Root,  A.  I.,  and  E.  R.  Root,  1908.  —  The  A  B  C  and  X  Y  Z  of  B< 

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219.  Sedgwick,  A.,  1909.  —  Text-book  of  Zoology,  vol.  3.     London. 


CHAPTER   XIII 
HISTORICAL  ZOOLOGY 

220.  Hertwig,  R.,  1902.  —  Manual  of  Zoology.     Translated  and  edi 
by  J.  S.  Kingsley.     New  York. 


BIBLIOGRAPHY 


319 


221.  Locy,  W.  A.,  1908.  —  Biology  and  its  Makers.     New  York. 

222.  Osborn,  H.  F.,  1894.  —  From  the  Greeks  to  Darwin.     New  York. 

223.  Parker,  T.  J.,  and  W.  A.  Haswell,  1897.  — Text-book  of  Zoology, 

vol.  2.     London. 

224.  Thompson,  J.  A.,  1899.    The  Science  of  Life.    London. 

CHAPTER  XIV 

GENERAL    CONSIDERATIONS    OF   ZOOLOGICAL   FACTS   AND 

THEORIES 

225.  Adams,  C.  C.,  1902.  —  Southeastern  United  States  as  a  Center  of 

Geographical  Distribution  of  Flora  and  Fauna.  Biol.  Bull., 
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!26.  Baldwin,  J.  M.,  1902.  —  Dictionary  of  Philosophy  and  Psychol- 
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227.  Bateson,  W.,  1902.  —  Mendel's  Principles  of  Heredity,  a  Defence. 
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128.  Beddard,  F.  E.,  1895.  —  A  Text-book  of  Zoogeography.  Cam- 
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229.  Bethe,  A.,  1898.  —  Diirfen  wir  den  Ameisen  u.  Bienen  Psychische 

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230.  Brooks,  W.  K.,  1883.  —  The  Law  of  Heredity.     Baltimore. 

231. ,  1899.  —  The  Foundations  of  Zoology.     New  York. 

232.  Conn,  H.  W.,  1903.  —  The  Method  of  Evolution.     New  York. 
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Chicago. 

234.  Cunningham,  J.  T.,  1892.  —  The  Evolution  of  Flat  Fishes.     Nat. 

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235.  Darwin,  Charles,  1890.  —  Journal  of  Researches  during  the  Voyage 

round  the  World  in  H.  M.  S.  Beagle,  2d  ed.     New  York. 
236. ,  1886.  —  On    the   Origin  of   Species   by    Means   of   Natural 

Selection.     New  York. 
237.  Delage,  Y.,  1884.  —  Evolution  de  la  Sacculine.     Arch.  zool.  exp6r. 

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238. ,   1003.  —  L'heredite  et    les  grands  proble'mes  de  la  biologic 

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239.  Eimer,  G.  H.  Th.,  1890.  —  Organic  Evolution.     London. 

Forel,  A.,  1904.  —  Ants  and  Some  Other  Insects.     Translated  by 

W.  M.  Wheeler.     Chicago. 
841.  Galton,  F.,    1897.  —  The   Average   Contribution  of  Each  Several 

Ancestor  to  the  Total  Heritage  of  the  Offspring.    Proc.  Roy.  Soc., 

London,  vol.  61,  pp.  401-413. 


320 


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242.  Haeckel,  Ernst,  1883.  —  The  Evolution  of  Man.  2  vols. 

243.  Headley,  F.  W.,  1900.  —  Problems  of  Evolution.     London. 

244.  James,  W.,  1890.  —  The  Principles  of  Psychology.     2  vols.     Nei 

York. 

245.  Jennings,  H.  S.,  1904.  —  Contributions  to  the  Study  of  the 

havior    of    Lower    Organisms.     Carnegie    Inst.     Pub.     Was 
ington. 

246.  ,  1906.  —  Behavior  of  the  Lower  Organisms.     New  York. 

247. ,   1908.  —  The  Interpretation  of  the  Behavior  of  the  Lo\ 

Organisms.     Science,  N.  S.,  vol.  27,  pp.  698-710. 
248.  Jordan,  D.  S.,  1905.  —  The  Origin  of  Species  through  Isolatic 

Science,  N.  S.,  vol.  22,  pp.  545-562. 
249. ,  and  V.  L.  Kellogg,  1907.  —  Evolution  and  Animal  life.     N< 

York. 

250.  Kellogg,  V.  L.,  1907.  —  Darwinism  To-Day.     New  York. 

251.  Loeb,  J.,  1900.  —  Comparative  Physiology  of  the  Brain  and  Coi 

parative  Psychology.     New  York. 

252. ,  1905.  —  Studies  in  General  Physiology,  2  pts.     Chicago. 

253. ,  1906.  —  The  Dynamics  of  Living  Matter.     New  York. 

254. ,  1907.  —  Concerning  the  Theory  of  Tropisms.     Journ.  E> 

Zool.,  vol.  4,  pp.  151-156. 

255.  Merriam,  C.  H.,  1898.  —  Life  Zones  and  Crop  Zones  of  the  Unit 

States.     Bull.  10,  Biol.  Survey,  U.  S.  Dep't  of  Agric. 

256.  Metcalf,  M.  M.,  1904.  —  Organic  Evolution.     New  York. 

257.  Montgomery,  T.  H.,  1906.  —  Analysis  of  Racial  Descent  in  Ai 

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258.  Morgan,  C.  L.,  1896.  —  Habit  and  Instinct.     London. 
259. ,  1908.  —  Animal  Behavior,  2d  ed.     New  York. 

260.  Morgan,  T.  H.,  1903.  —  Evolution  and  Adaptation.     New  York. 
261. ,  1907.  —  Experimental  Zoology.     New  York. 

262.  Muller,  F.,  1864.  —  Fur  Darwin.     Leipzig. 

263.  Osborn,  H.  F.,  1908.  —  The  Four  Inseparable  Factors  of  Evoh 

tion.     Science,  N.  S.,  vol.  27,  pp.  148-150. 

264.  Plate,  L.,  1903.  —  Ueber  die  Bedeutung  der  Darwin'schen   Sel( 

tionsprinzip. 

265.  Romanes,  J.  G.,  1892-1897.  —  Darwin  and  after  Darwin.     3  vc 

London. 

266.  Ruthven,  A.  G.,  1908.  —  Variations  and  Genetic  Relationships 

the  Garter  Snakes.     Bull.  61,  U.  S.  Nat.  Mus. 

267.  Stevens,  W.  C.,  1902.  —  Introduction  to  Botany.     Boston. 

268.  Thomson,  J.  A.,  1896.  —  The  Study  of  Animal  Life.     New  Yor 
269. ,  1908.  —  Heredity.     New  York. 


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321 


^70.  Tower,  W.  L.,  1906.  —  Evolution  in  Chrysomelid  Beetles  of  the 
Genus  Leptinotarsa.     Carnegie  Inst.  Pub.     Washington. 

271.  Van  Beneden,  E.,  1889.  —  Animal  Parasites  and  Messmates. 

272.  Vries,  H.  de,  1905.  —  Species  and  Varieties,  their  Origin  by  Muta- 

tion.    Chicago. 

273.  Wallace,  A.  R.,  1876.  —  The  Geographical  Distribution  of  Animals. 

London. 

274. ,  1881.  —  Island  Life.     London. 

275. ,  1903.  —  Darwinism.     London. 

276.  Washburn,  M.  F.,  1908.  —  The  Animal  Mind.     New  York. 

277.  Wasmann,  E.,    1903.  —  Instinct  and  Intelligence  in  the  Animal 

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278.  Weismann,  A.,  1904.  —  The  Evolution  Theory.     2  vols.     London. 

279.  Weysse,  A.  W.,  1904.  —  A  Synoptic  Text-book  of  Zoology.     New 

York. 

280.  Whitman,  C.  O.,  1906.  —  The  Problem  of  the  Origin  of  Species. 

Proc.  Cong.  Arts  and  Sci.,  Universal  Exposition,  St.  Louis,  voL 
5,  pp.  41-58. 


GLOSSARY 


a  as  in  mat,  man. 
a  as  in  mate,  dale. 
a  as  in  far,  father. 
a  as  in  fall,  law. 

e  as  in  met,  pen. 
e  as  in  free,  meet. 
e"  as  in  her,  fern. 

i  as  in  tin,  it. 
i  as  in  tie,  pie. 


KEY  TO  PRONUNCIATION 

o  as  in  not,  on. 

5  as  in  note,  soul. 

6  as  in  move,  spoon. 
6  as  in  nor,  song. 

u  as  in  tub,  son. 
u  as  in  due,  mute. 


I  conceal),  the 


abdomen,  abdo'men   (L.  abdomen,  the  belly  —  from 

lower  belly. 
absorption,  absdrp'shon  (L.  absorbere,  swallow  down  anything),  the  process 

of  taking  up,  as  by  chemical  or  molecular  action. 
accretion,  a  kre'shon  (L.  ad,  to  ;    cretum,  to  grow),  growth  by  external  addi- 

tion of  new  matter. 
achromatin,  a  kro'ma  tin  (Gr.  a,  without  ;   chroma,  color),  the  non-staining 

substance  of  the  nucleus. 
accelomate,  a  se'lo  mat  (Gr.  a,  without  ;   koilos,  hollow),  without  a  true  body 

cavity  or  coelom. 
afferent,  afferent  (L.  off  'era,  I  bring  to),  conveying  from  the  surface  to  the 

center. 

ameba,  a  me'ba  (Gr.  amoibos,  change),  a  genus  of  rhizopodous  Protozoa. 
amitosis,  a  mi  to'  sis  (Gr.  «,  without  ;   mitos,  a  thread),  direct  nuclear  division 

without  the  formation  of  chromosomes  and  amphiaster. 
amphiaster,  am  'ft  aster  (Gr.  amphi,  around;   aster,  a  star),  the  achromatic 

figure  formed  in  mitotic  cell  division,  consisting  of  two  asters  connected 

by  a  spindle. 
anabolism,  an  ab'o  lizm  (Gr,  anabote,  a  throwing  up),  constructive  metab- 

olism. 
analogous,  a  naVogus  (Gr.  ana,  similar  to;   logos,  proportion),  like  in  function, 

but  not  in  structure. 

*  The  Century  Dictionary  and  Cyclopedia,  the  Rev.  James  Stormonth's  Manual 
of  Scientific  Terms,  and  E.  B.  Wilson's  Cell  in  Development  and  Inheritance  have 
been  invaluable  in  preparing  this  glossary- 

$23 


324 


GLOSSARY 


anaphase,  an'afdz  (Gr.  ana,  back  or  again),  the  later  period  of  mitosis 

during  the  divergence  of  the  daughter  chromosomes. 
anatomy,  a  nat'omi  (Gr.  anatemno,  to  cut  up),  the  study  of  the  structure  of 

organisms  as  made  out  by  dissection. 
Annelida,  a  nel'idd  (L.  annellus,  a  little  ring;   Gr.   eidos,  resemblance),  a 

phylum  of  animals  having  bodies  made  up  of  many  small  rings. 
antenna,  an  ten! a  (L.  antenna,  a  sail  yard),  the  jointed  feelers  upon  the 

heads  of  insects  and  crustaceans. 
antennule,  anten'til  (L.  dim.  of  antenna),  the  smaller  pair  of  antennae  in 

the  crustaceans. 

anus,  d'nus  (L.  anus,  a  ring),  the  orifice  through  which  the  refuse  of  diges- 
tion is  voided. 

Apis,  d'pis  (L.  apis,  a  bee),  a  genus  of  bees. 
archenteron,  arken'teron   (Gr.  arche,  beginning;    enferon,  intestine),  the 

primitive  digestive  cavity. 
arthrobranchia,  dr  thro  brang'ki  a  (Gr.  arthron,  a  joint ;   brangchia,  the  gill 

of  a  fish),  part  of  the  respiratory  system  of  the  crayfish. 
Arthropoda,  dr throp'od'd  (Gr.  arthron,  a  joint  ;  pous,  foot),  a  phylum  of 

animals  having  bodies  composed  of  segments,  some  or  all  of  which  bear 

jointed  appendages. 
Ascaris,  as'ka  ris  (Gr.  askaris,  a  worm  in  the  intestines),  a  genus  of  round 

worms. 
asexual,  asek'sual   (Gr.  a,   without;    and   sexual},  applied   to   modes   of 

reproduction  in  which  the  sexes  are  not  concerned. 
assimilatii)n;  asim  ild'shon  (L.  assimilo,  to  make  like),  the  conversion  of 

digestec1    ood  into  living  protoplasm. 
aster,  as'ter  (kGr.  aster,  a  star),  the  star-shaped  structure  surrounding  the  cen- 

trosome. 
auditory,  flditd ri  (L.  auditor,  a  hearer),  pertaining  to  the  sense  of  hearing. 

behavior,  behdv'yor  (be,  about;  habban,  hold,  have),  the  sum  total  of  all 
the  various  movements  of  an  animal. 

bilateral,  bilaferal  (L.  bis,  twice;  lateris,  of  a  side),  having  the  sides  sym- 
metrical. 

biogenesis,  btojen'esis  (Gr.  bios,  life  ;  genesis,  origin),  the  genesis  of  living 
beings  from  living  beings. 

biology,  bi otoji  (Gr.  bios,  life  ;  logos,  discourse),  the  science  of  life  and  liv- 
ing things. 

biramous,  fora'mus  (L.  bi,  two;  ramus  a  branch),  dividing  into  two 
branches. 

blastOC03ls  blas!tosel  (Gr.  blastos,  a  bud  ;  koilos,  a  hollow),  the  cavity  of  the 
blastula. 


GLOSSARY 


325 


blastoderm,  bias* to  derm  (Gr.  blastos,  a  bud  ;   derma,  skin),  the  layer  of  cells 

forming  the  wall  of  the  blastula. 
blastomere,  bias1  to  mer  (Gr.  blastos,  a  bud  ;  meros,  a  part) ,  a  term  applied  to 

a  cell  during  cleavage  of  the  egg. 
blastopore,  bias' to por  (Gr.  blaslos,  a  bud  ;  poros,  pore),  the  opening  of  the 

gastrula,  the  primitive  mouth, 
blastula,  bias' til  Id  (dim.  of  Gr.  blastos,  a  bud),  an  embryo  consisting  of  a  sac 

formed  of  a  single  layer  of  cells. 

botany,  bot'a  ni  (Gr.  botane,  a  plant),  the  science  which  treats  of  plants. 
branchial,  branchial  (L.  branchiae,  gills),  pertaining  to  the  gills, 
buccal,  buk'al  (L.  bucca,  a  cheek),  pertaining  to  the  cheek  or  mouth. 

Cambarus,  kam'ba  rus  (L.  camarus,  a  sea  crab),  a  genus  of  crayfishes. 

capillary,  kap'i  13,  ri  (L.  capillus,  hair),  a  hairlike  tube. 

carapace,  kar'a  pas  (Gr.  karabos,  a  crustaceous  animal  like  the  crab),  the 

shield  covering  the  cephalothorax  of  the  crayfish. 
carbohydrate,  kar  bo  hi1  drat   (L.    carbo,   a   coal  ;    Gr.   hudor,  water),   an 

organic  body  containing  6  carbon  atoms  or  some   multiple  of  6,  and 

hydrogen  and  oxygen  in  the  proportion  in  which  they  form  water  (H^O). 
cardiac,  kdr'di  ak  (Gr.  kardia,  the  heart),  pertaining  to  the  heart, 
caudal,  kd'dal  (L.  cauda,  a  tail),  having  a  position  or  relation  toward  the 

tail  when  compared  with  some  other  part. 
Cell,  set  (L.  cella,  a  store-room),  a  mass  of  protoplasm  containing  a  nucleus  ; 

the  unit  of  structure  of  the  Metazoa. 
centrosome,  senffrosom   (Gr.  centron,  center;    soma,  body),  a  body  found 

at  the  center  of  the  aster  of  the  amphiaster  during  mitotic  cell  division. 
Cephalic,  sefdfik  (Gr.   kephale,  the    head),  having   a   position   or   relation 

toward  the  head  when  compared  with  some  other  part, 
cephalothorax,  sefa  Id  tho'raks  (Gr.  kaphale,  the  head  ;   thorax,  the  thorax), 

the  (coalesced)  head  and  thorax  of  certain  arthropods, 
cervical,  ser'vi  kal  (L.  cervix,  the  neck),  of  or  pertaining  to  the  neck. 
Chela,  ke'ld  (Gr.  kele,  a  claw),  the  pair  of  pinchers  that  terminates  some  of 

the  appendages  of  certain  crustaceans. 
cheliped,  ke'liped  (Gr.  kele,*.  claw;   L.  pes,  foot),  the  chelate  walking  legs 

of  the  crayfish  and  other  crustaceans. 
Chemotropism,  kern  ot'ro  pism  (Gr.  chemeia,  a  mingling;  trope,  a  turning), 

the  reaction  of  an  animal  to  a  chemical, 
chitin,  kit  tin  (Gr.  chiton,  a  coat  of  mail),  the  organic  substance  forming  the 

exoskeleton  of  arthropods  and  certain  other  animals. 
ChlorogOgen,  kid' r 5 go' j en  (Gr.  chloros,  grass-green),  (see  p.  169). 
Chlorophyll,  klo'rojil  (Gr.  chloros,  grass-green  :   phullon,  a  leaf),  the  green 

coloring  matter  of  plants. 


326  GLOSSARY 

choanocyte,  ko' a  no  sit  (Gr.  choane,  a  funnel;   kutos,  a   cavity),  a  collared 

cell  of  the  sponge. 
chromatin,  krd'ma  tin  (Gr.  chroma,  color),  the    deeply  staining  substance 

of  the  nuclear  network  and  of  the  chromosomes,  consisting  of  nuclein. 
Chromosome,    kro'mo  som    (Gr.   chroma,    color;    soma,   body),   the    deeply 

staining  bodies  into  which  the  chromatic  nuclear  network  resolves  itself 

during  mitosis. 
Chromotropism,  kro  nibt'ropizm  (Gr.  chroma,  color;  trope,  a  turning),  the 

reaction  of  an  animal  to. color. 
cilium,  sit i  um  (L.  cilium,  an  eyelid  with  the  hairs  growing  on  it),  a  minute 

hairlike  process  of  a  cell. 
Class,  kl'as  (L.  classis,  an  assembly  of  people),  a  number  of  animals  regarded 

as  a  collective  unit  because  of  the  presence  of  certain  common  characters. 
cleavage,  kle'vdj,  th^mvision  of  the  fertilized  egg. 
clitellum,  klitel'um  (L.  clitellx,  a  pack  saddle),  a  glandular   ring  around 

the  body  of  the  earthworm. 
cnidoblast,  ni'do  blast  (Gr.  knide,  a  nettle  ;   blastos,  a  bud),  a  cell  in  which  a 

nematocyst  is  developed.    . 
cnidocil,  m'do  sil(Gr.  knide,  a  nettle;   L.  ciliiim,  an  eyelid),  a  stiff  hairlike 

process  projecting  from  a  cnidoblast. 
coagulate,  ko  ag'ti  Idt  (L.   coagulare,  curdle),  change  from  a  fluid   into   a 

curdlike  or  thickened  mass. 
Ccelentera,  se  len'te  r'd  (Gr.  koilos,  hollow;   enteron,  intestine),  a  phylum  of 

animals  with  a  single  body  cavity  which  opens  by  a  single  orifice,  the 

mouth. 
CCelom,  se'lom  (Gr.  koilos,  hollow),  the  true  body  cavity,  lying  between  the 

digestive  tract  and  the  body  wall,  and  lined  with  epithelium, 
coelomate,  se  Id' mat  (Gr.  koilos,  hollow),  having  a  coelom. 
commensalism,  ko  men'salizm  (L.  com,  together;   mensa,  table),  the  living 

together  of  two  species  of  animals  or  plants,  but  neither  at  the  expense  of 

the  other. 

conjugation,  konjo gd'shon  (L.  conjugatum,  to  unite),  a  temporary  or  perma- 
nent union  of  two  cells  for  reproduction. 

contractility,  kon  traktil'i  ti  (L.  contractus,  a  drawing  together),  the  prop- 
erty or  force  by  which  bodies  shrink  or  contract. 
copulation,  kop  n  Id'shon  (L.   copulare,  unite),   the   active   transmission   of 

sperm  from  the  male  to  the  female. 

cortical,  kbr'ti  kal  (L.  cortex,  bark,  rind),  belonging  to  the  external  covering, 
cuticle,  ku'ti  kl  (L.  cuticula,  dim.  of  cutis,  the  skin),  the  outermost  covering 

of  the  body  of  certain  animals. 
cyclosis,  st  kid' sis  (Gr.  kuklos,  a  circle),  applied  to  currents  within  the  bodies 

o*  Protozoa 


GLOSSARY  327 

Cyst,  sist  (Gr.  kustis,  a  bladder),  a  term  applied  to  certain  Protozoa  when 
they  surround  themselves  with  a  wall  and  pass  into  a  resting  stage. 

cytopharynx,  si'to  far' ingks  (Gr.  kutos,  a  vessel;  pharungx,  gullet),  a  tube 
leading  from  the  bottom  of  the  oral  groove  into  the  body  of  a  Paramecium. 

cytoplasm,  si'to  plazm  (Gr.  kutos,  a  vessel  ;  plasma,  anything  formed),  the 
substance  of  the  cell  body  as  opposed  to  that  of  the  nucleus. 

diastase,  dT'a  stas  (Gr.  diastasis,  a  separation),  a  substance  which  has  the 
property  of  converting  starch  into  sugar. 

diastole,  di  as' to  le  (Gr.  diastole,  separation),  the  period  of  time  during  which 
a  rhythmically  pulsating  vessel  is  relaxed  or  dilated. 

digestion,  dijes'tyon  (L.  digestio,  the  dissolving  of  food),  the  process  of  pre- 
paring food  for  absorption. 

dioecious,  dte'shus  (Gr.  dis,  twice  ;  oikos,  a  house),  h^gpg  the  sexes  distinct, 
applied  to  species  that  consist  of  male  and  female  individuals. 

diploblastic,  dip  lo  bias1  tik  (Gr.  diploos,  double;  blastos,  a  bud),  having  two 
germinal  layers. 

dissimilation,  di  sim  i  Id 'shon  (L.  dissimilare,  make  unlike),  the  processes 
by  which  protoplasm  is  broken  down  into  simpler  products. 

distal,  dis'tal  (L.  disto,  I  stand  apart),  situated  away  from  the  place  of  attach- 
ment. 

dorsal,  dor1  sal  (L.  dor  sum,  the  back),  of  or  pertaining  to  the  back. 

ecdysis,  ek  di  sis  (Gr.  ekdusis,  emerging),  shedding  or  molting  the  external 
covering. 

ectoderm,  ek' to  derm  (Gr.  ektos,  outside  ;   derma,  skin),  the  outer  germ  layer. 

ectosarc,  ek'to  sark  (Gr.  ektos,  outside;  sarx,  flesh),  the  outer  layer  of  cer- 
tain Protozoa. 

efferent,  efe  rent  (L.  ef  for  ex,  out ;  fero,  I  carry),  conveying  outward. 

egestion,  ejes'chon  (L.  egestio,  void),  the  act  of  voiding  the  refuse  of  digestion. 

electrotropism,  e  lek'tro  tro  pizm  (Gr.  elektron,  amber;  trope,  a  turning),  re- 
action  to  the  electric  current. 

embryo,  em'bri  o  (Gr.  en,  in  ;  bruo,  bud),  the  early  stage  of  an  animal  when 
it  is  within  the  egg  membrane. 

endopodite,  endop'ddtt  (Gr.  endon,  within;  pous,  a  foot),  the  r'nner  of  the 
two  main  divisions  of  a  typical  crustacean  appendage. 

endosarc,  en1  do  sark  (Gr.  endon,  within  ;  sarx,  flesh),  the  inner  protoplasm 
of  certain  Protozoa. 

enteron,  en'teron  (Gr.  enteron,  intestine),  the  digestive  tract  that  is  primi- 
tively derived  from  the  entoderm. 

entoderm,  en* to  derm  (Gr.  endon,  within  ;   derma,  skin),  the  inner  germ  layer. 

enzyme,  en'zim  (Gr.  en,  in  ;   zume,  leaven),  an  unorganized  ferment. 


GLOSSARY 


epigenesis,  epijen'esis  (Gr.  epi,  upon;  genesis"),  a  theory  of  development 

(see  p.  272). 
epipodite,  epip'o  dlt  (Gr.  epi,  upon;  pous,  foot),  a  process  developed  upon 

the  basal  joint  of  some  of  the  appendages  of  certain  crustaceans. 
epithelium,  epithelium  (Gr.  epi,  upon;   thallo,  I  grow),  the  layer  of  cells 

forming  the  surface  of  all  the  internal  membranes  of  the  body. 
evolution,  evo  lu'shon  (L.  e,  out;   volvo,  roll),  a  theory  of  development  (see 

p.  291). 
excretion,  ekskre'shon  (L.  ex,  out;   cretus,  separated),  the   elimination  of 

useless  or  harmful  substances  from  the  body. 
exopodite,  tksop'odlt  (Gr.  exo,  outside;  pous,  foot),  the  outer  of  the  two 

main  divisions  of  a  typical  crustacean  appendage. 
exoskeleton,  ek  so  skel'  e  ton  (Gr.  exo,  outside;   skeleton},  any  structure  pro- 

duced by  the  hardening  of  the  integument. 


s  (L.ftzx,  dregs),  undigested  particles  cast  out  by  an  animal. 
family,  fa  m'ili  (L.  familia,  a  household),  the  name  of  a  group  above  a 

genus  and  below  an  order. 
fauna,  fd'na  (L.  Faunus,  one  of  the  gods  of  the  fields),  all  the  animals  pe- 

culiar to  a  country,  area,  or  period. 
ferment,  fer'ment  (  L.  fermentum,  leaven),  a  substance  which  transforms  an 

organic  substance  into  new  compounds. 
fertilization,  fl-r'ti  li  za'shon  (L.  fertilis,  fruitful),  the  union  of  a  sperma- 

tozoon with  an  egg. 
flagellum,  flajel'um   (L.  flagellum,  a  whip),  the   whiplike   appendage  of 

many  Protozoa. 

foliaceous,  fo  li  d'shius  (L,.  folium,  leaf)    resembling  a  leaf. 
function,  fungk'shon  (L.  functio,  performance),  the  mode  of  action  proper 

to  an  organ  or  structure. 

gamete,  gam'cf  (Gr.  gamete,  a  wife),  a  reproductive  cell  which  unites  with 

another  reproductive  cell  to  form  a  zygote. 
ganglion,  gang'gli  on  (Gr.  gangglion,  a  tumor  under  the  skin  near  a  tendon), 

a  mass  of  nervous  tissue  containing  nerve  cells  and  giving  rise  to  nerve 

fibers. 

gastric,  gas'trik  (Gr.  gaster,  stomach),  of  or  pertaining  to  the  stomach. 
gastrolith,  gas'tro  lith  (Gr.  gaster,  stomach  ;   lithos,  stone),  a  stony  concre- 

tion in  the  stomach  of  certain  crustaceans. 
gastrula,  gas'tro  la  (dim.  of  Gr.  gaster,  stomach),  an  embryo  consisting  of 

two  germ  layers  inclosing  a  central  cavity. 
genital,  jen'ital  (L.  genitalis,  serving  to  beget),  pertaining  to  the  organs  of 

generation. 


GLOSSARY  329 

genus,  je'nus  (L.  genus,  race),  a  group  containing  one  or  more  species, 
geotropism,  jeot'ropizm    (Gr.  gea,    earth;    trope,  a   turning),   reaction   to 

gravity. 

germ  cell,  jerm'  sel  (L.  germen,  bud  ;   cella,  store  room),  a  reproductive  cell, 
germ  layer,  jerm' Id'er  (L.  germen,  bud),  one  of  the  fundamental  embryonic 

membranes  from  which  the  organs  of  the  body  arise, 
germ  plasm,  jerm'pla'zm  (L.  germen,  bud ;   Gr.  plasma,  a  thing  molded) 

the  protoplasm  of  the  germ  cells. 
gullet,  gufet  (Y.goule,  mouth),  something  resembling  the  throat  in  shape 

position,  or  functions. 
gynandromorph,  ji  nan'dro  mdrf  (Gr.  gunandros,  of  doubtful  sex ;  morpht. 

form),  an  animal  with  both  male  and  female  characters. 

haemoglobin,  hem  oglo'bin  (Gr.  haima ;    L.  globus,  ball),  the  red  coloring 

matter  in  the  blood  of  certain  animals. 

hepatic,  hepat'ik  (Gr.  hepar,  liver),  of  or  pertaining  in  any  way  to  the  liver, 
heredity,  he  retfiti  (L.  heres,  an  heir),  the  resemblance  of  child  to  parent. 
hermaphrodite,  her  maf'ro  dit  (Gr.  Hermes,  the  god  mercury;   Aphrodite,  the 

goddess  Venus),  an  animal  possessing  the  reproductive  organs  of  both 

male  and  female. 
heterOHiOrphosis,  het'e  ro  morf'o  sis  (Gr.  heteros,  different;   morphe,  form),  in 

regeneration,  the  production  of  a  new  part  unlike  that  removed, 
histology,  his  tot 5 ji  (Gr.  histos,  tissue;   logos,  discourse),  the  study  of  the 

microscopic  structure  of  tissues, 
holoblastic,  hold  bias1  tik  (Gr.  holes,  whole;   blastos,  germ),  applied  to  eggs 

with  total  cleavage, 
holophytic,  hold  fit1  ik  (Gr.  holos,  whole;  phuton,  a  plant),  resembling  a  plant 

in  mode  of  nutrition. 

holozoic,  hoi  o  zo'ik  (Gr.  holos,  whole  ;   zoon,  an  animal),  resembling  an  ani- 
mal in  mode  of  nutrition. 
homologous,  homoVogus  (Gr.  homos,  VAae;   logos,  speech),  having  the  same 

relative  position  or  structure,  i.e.  anatomically  similar. 
Hydra,  hi  dr'd  (L.  hydra,  a  water  snake),  a  genus  of  fresh  water  polyps, 
hypodermis,  hy po  der'mis  (Gr.  hupo,  under  ;   derma,  skin),  the  layer  of  cells 

just  below  the  cuticle  of  certain  animals, 
hypostome,  hi'postom  (Gr.  httpo,  under  ;  stoma,  a  mouth),  a  structure  near 

the  mouth  of  certain  coelenterates  and  crustaceans. 

ingestion,  injes'chon  (L.  ingestus,  poured  into),  the  introduction  of  food  into 

the  body. 
interstitial,  in  ter  stish' al  (L.  inter,  between  ;  sisto,  I  stand),  pertaining  to 

pr  situated  in  an  intervening  space. 


330  GLOSSARY 

intracellular,  intrd  seVu  lar  (L.  intra,  within),  existing  or  done  inside  of  a 
cell. 

intussusception,  in'tususep'shon  (L.  intus,  within;  susceptus,  taken  up), 
the  addition  of  new  particles  among  the  preexisting  particles  of  proto- 
plasm. 

irritability,  ir'ita  biPiti  (L.  irritare,  excite),  the  property  of  responding  to 
stimuli. 

karyoplasm,  kar'i  o  plazm  (Gr.  karuon,  nucleus ;  plasma,  a  thing  formed), 

the  substance  of  the  nucleus. 
karyosome,  kar'iosom  (Gr.  karuon,  nucleus ;  soma,  body),  nucleoli  which 

stain  with  nuclear  dyes. 
katabolism,  katatfolizm  (Gr.  katabole,  a  throwing  down),  the  process  by 

which  protoplasm  breaks  down  into  simpler  products. 

larva,  lar'va  (L.  larva,  a  ghost  or  mask),  the  young  of  any  animal  which 

during  its  development  is  unlike  its  parent. 
lateral,  lat'e  ral  (L.  latus,  side],  of  or  pertaining  to  the  side. 
Lumbricus,  lum  bri'kus  (L.  lumbricus,  earthworm),  a  genus  of  worms. 

macrogamete,  mat?rdgam!et  (Gr.  makros,  long;  gamete,  wife),  a  large  re- 
productive cell,  the  egg. 

macromere,  mak'ro  mer  (Gr.  makros,  long ;  mt-ros,  a  part),  the  larger  cells 
in  the  early  embryonic  stages  of  certain  animals. 

macronucleus  mak  ro  nfi'kle  us  (Gr.  makros,  long  ;  nucleus),  a  large  nucleus 
in  certain  Protozoa. 

maturation,  matil  ra'shon  (L.  maturare,  ripen),  the  ripening  of  the  egg  by 
the  formation  of  polar  bodies. 

maxilla,  mak  sit  a  (L.  maxilla,  a  jaw),  an  appendage  near  the  mouth  of 
arthropods. 

maxilliped,  mak sil'i fed  (L.  maxilla,  a  jaw  ;  pedes,  foot),  a  footlike  append- 
age near  the  mouth  of  certain  arthropods. 

medusa,  tnedu'sa  (L.  Medusa,  a  mythological  woman  whose  hair  was  turned 
into  snakes),  a  jellyfish,  or  reproductive  zooid  of  certain  coelenterates. 

meroblastic,  mer  o  bias1  tik  (Gr.  meros,  a  part  ;  blastos,  a  germ),  applied  to 
eggs  only  part  of  which  are  cut  up  into  cells  during  cleavage. 

mesoderm,  me' so derm  (Gr.  mesos,  middle;  derma,  skin),  the  middle  germ 
layer  of  triploblastic  animals. 

mesoglea,  mesogle'a  (Gr.  mesos,  middle  ;  gloia,  glue),  the  gelatinous  sub- 
'stance  between  the  ectoderm  and  entoderm  of  coelenterates. 

metabolism,  metaVolizm  (Gr.  metabole,  change),  the  processes  connected 
with  the  manufacture  and  destruction  of  protoplasm. 


GLOSSARY 


331 


metamere,  mefa  mer  (Gr.  meta,  after  ;   meros,  a  part),  one  of  a  longitudinal 

series  of  parts  which  are  serially  homologous  with  one  another, 
metamorphosis,   meta  mor'fo  sis   (Gr.   mefa,   change  ;     morphe,   shape),   a 

marked  change  in  form  or  function. 
metaphase,  mettafdz  (Gr.  mefa,  after),  the  middle  stage  of  mitosis  during 

which  occurs  the  splitting  of  the  chromosomes  in  the  equatorial  plate. 
metaplasm,  met'aplazm  (Gr.  meta,  after;  plasma,  a  thing  formed),  applied 

to  lifeless  inclusions  in  protoplasm. 

Metazoa,  metazo'a  (Gr.  meta,  after;    zoon,  an    animal),   many-celled   ani- 
mals. 
microgamete,  mlkrogam'et  (Gr.    mikros,  small ;    gamete,   a  wife),  a  small 

reproductive  cell,  the  spermatozoon. 
micromere,  mi'kromer  (Gr.  mikros,  small;   meros,  a  part),  the  smaller  cells 

in  the  embryonic  stages  of  certain  animals, 
micronucleus,  ml krd  nu'kle  us  (Gr.  mikros,  small ;   nucleus},  the  smaller  of 

two  nuclei  in  certain  Protozoa. 

mitosis,  mi  to' sis  (Gr.  mitos,  a  thread),  indirect  nuclear  division, 
monoecious,  mo  ne'shus  (Gr.  monos,  one  ;   oikos,  a  house),  having  both  male 

and  female  sexual  organs. 
morphology,  mbrfol'oji  (Gr.  morphe,  form;   logos,  description),  the  science 

of  form  and  structure. 
moult,  molt  (L.  mutare,  change),  the  process  of  casting   off  tegumentary, 

cuticular,  or  exoskeletal  st-.uctures. 
mutation,  mu  td'shon  (L.  mutare,  change),  a  theory  of  the  origin  of  species 

(see  p.  295). 

nematocyst,  nem'atosist  (Gr.  nema,  thread;  kustis,  a  bag),  a  structure  con- 
taining a  thread,  characteristic  of  coelenterates. 

nephridium,  nefricfium  (Gr.  nephros,  a  kidney),  one  of  the  excretory 
organs  of  the  earthworm. 

neuron,  nii'ron  (Gr.  neuron,  a  nerve),  a  nerve  cell  with  all  its  prolongations. 

nucleolus,  nukle'd lus  (L.  nucletts,  a  small  nut),  a  deeply  staining  body  often 
present  within  a  nucleus. 

nucleus,  nt't'kleus  (L.  nucleus,  a  small  nut),  the  dynamic  center  of  a  cell. 

nutriment,  nu'triment  (L.  nutrire,  nourish),  that  which  promotes  the 
growth  or  repairs  the  waste  of  animal  bodies. 

nutrition,  nutrish'on  (L.  nutrire,  nourish),  the  processes  by  which  organ- 
isms take  in  food  and  add  it  to  their  living  tissues. 

ocellus,  osel'us  (L.  ocellus,  a  little  eye),  a  simple  eye  of  an  insect. 
oesophagus,  e  sofa  gus  (Gr.  oisophagos,  the  gullet),  the  canal  through  which 
food  and  drink  passes  to  the  stomach. 


332 


GLOSSARY 


ommatidmm,  om  atid'ium  (Gr.  dim.  of  omma,  eye),  part  of  the  compound 

eye  of  an  arthropod, 
ontogeny,  on  tofeni  (Gr.  on,  being;  gennao,  I  produce),  the  development  of 

an  individual  organism. 

OOCyte,  5'osTt  (Gr.  oon,  an  egg),  a  stage  in  the  maturation  of  an  egg. 
OOgenesis,   oojen'esis    (Gr.  oon,   an    egg;   genesis,  origin),  the  origin  and 

development  of  the  egg. 
OOgonium,  oogo'nium    (Gr.    oon,  an  egg;  gonos,  generation),  an  egg  just 

before  maturation  takes  place. 
organ,  or'gan  (L.  orgamtm,  an  instrument),  a  part  of  a  organism  having  a 

specific  function. 

orthogenesis,  or  thojen'e sis  (Gr.  orthos,  straight  ;  genesis),  a  theory  of  evo- 
lution (see  p.  294). 
osculum,  os1 ku  lum  (L.  osculum,  a  little  mouth),  an  opening  through  which 

water  is  expelled  from  a  sponge. 
Ostium,  os'ti  um  (L.  ostium,  an  opening),  an  opening  in  various  organs  of  the 

body. 
ovary,   o'va  ri  (L.  ovum,  an  egg),  the  organ  of  the  female  in  which  eggs 

develop. 

Oviduct,  o'vidukt  (L.  ovum,  an  egg ;   ductus,  led),  the  duct  of  the  ovary. 
ovum,  o'vum  (L.  ovum,  an  egg),  an  egg. 
oxidation,  ok  si  da'  shon  (Gr.  oxus,  sharp),  the  combination  of  a  substance 

with  oxygen. 


pa'le  ontoPoji  (Gr.  palaios,  ancient  ;   onto,  beings;   logos,  dis- 
course), the  science  of  fossil  organisms. 
parasite,  par1  a  sit  (Gr.  parasitos,  one  who   eats  at   another's  expense),  an 

animal  that  lives  in,  on,  or  at  the  expense  of  another  animal, 
parietal,  pa  ri'e  tal  (L..  paries,  wall),  pertaining  to  the  walls. 
parthenogenesis,  par' the  no  jen'e  sis    (Gr.   parthenos,  a   virgin  ;   gennao,   I 

produce),  reproduction  by  means  of  unfertilized  eggs. 
pellicle,  pel'ikl  (L.  pelliciila,  a  small  skin),  a  thin  outer  covering  of  skin  or 

cuticle, 
psricardium,  perikdr'dium  (Gr.  peri,  round  about  ;   kardia,  the  heart),  a 

membranous  sac  surrounding  the  heart. 
peristaltic,  peristaFtik    (Gr.  peristaltikos,   drawing   together    all   round), 

applied  to  the  waves  of  contraction  running  down  the  alimentary  canal. 
peristome,  per'istom   (Gr.  peri,  round   about;   stoma,  a   mouth),  the  part 

which  surrounds  the  mouth  or  oral  opening. 
peritoneum,  per'itone'um    (Gr.  peritonaion,  what    is    stretched   round   or 

over),  the  membrane  lining  the  body  cavity  and  covering  most  of  the 

organs  in  the  abdomen. 


GLOSSARY  333 

pharynx,  far'ingks  (Gr.  pharungx,  the  gullet  or  windpipe),  a  part  of  the 

alimentary  canal  between  the  mouth  and  oesophagus. 

photosynthesis,  fo  to  sin' the  sis    (Gr.  phos,   light;    synthesis,  a  putting   to- 
gether), the  manufacture  of  starch  by  chlorophyll  in  the  presence  of 

light. 
phototropism,  fotofropizm  (Gr.  phos,  light  ;   trope,  a  turning),  reaction  to 

light. 
phylogeny,  fl loj'eni  (Gx.phylon,  a  tribe;  gennao,  I  produce),  the  study  of 

the  ancestral  history  of  organisms. 
phylum,  fl'lum  (Gr.  phylon,  a  tribe),  any  primary  division  of  the  animal  or 

vegetable  kingdom, 
physiology,  fiz  i ol'oji  (Gr.  phusis,  nature;    logos,  discourse),  the  study  of 

the  functions  of  living  things. 
plasma,  plas'ma  (Gr.  plasma,  a  thing  formed),  protoplasm  ;   the  liquid  part 

of  the  blood, 
plasm osome,  plas'mdzom    (Gr.  plasma,  a   thing  formed  ;    soma,  body),  a 

nuclear  constituent  distinguished  by  its  affinity  for  plasma-stains. 
plastid, plas 'tid  (Gr.  plasso,  I  form  or  mold),  a  permanent  cell  organ  other 

than  nucleus  and  centrosome. 

pleopod,  pie' o pod  (Gt.pleein,  swim;  pous,  foot),  one  of  the  abdominal  ap- 
pendages of  a  crustacean  ;   swimmeret. 
polar  bodies,  two  minute  cells  segmented  off  from  the  ovum  before  union  of 

the  germ  nuclei. 
pollen,  pol'en  (L.  pollen,  fine  flour),  the  fertilizing  powder  contained  in  the 

anthers  of  flowers. 
pronucleus,  pronu'klsus    (L.    pro,   before;    nucleus},   a   male   or    female 

nucleus  during  fertilization. 

prophase,  pro'fdz  (Gr.  pro,  before),  the  early  period  of  mitosis. 
propolis,  prop 'o Us  (Gr.  pro,  before  ;    polls,  city),  a  substance  collected  by 

bees  to  stop  up  crevices  in  the  hive, 
prostomium,  prosto'mium  (Gr.pro,  before  ;   stoma,  mouth),  the  region  in 

front  of  the  mouth. 
proteid,  pro'le  id  (Gr.  protos,  first),   a  nitrogenous  substance  found  in   the 

bodies  of  plants  and  animals, 
proteus,  pro'te  us    (a   sea  god  who    had   the   power  of  assuming  different 

shapes),  the  specific  name  of  an  ameba. 
protoplasm,  pro'toplazm   (Gr.  protos,  first;    plasma,  a  thing  formed),  the 

essential  substance  of  the  bodies  of  organisms. 
protopodite,/r<7/0/'<?^7  (Gr.  protos,  first;  pous,  a  foot),  the  basal  segment 

of  a  crustacean  appendage. 
Protozoa,  pro  to  zd1 'a   (Gr.  protos,  first;   zoon,  animal),  a  phylum  of  animals 

(see  p.  80). 


334  GLOSSARY 

proximal,    AroVsimal   (L.  proximus,   nearest),  situated  near  the  place  of 

attachment. 
pseudopodium,  sii  do  po'di  um   (Gr.  pseudes,  false  ;  pous,  foot),  a  temporary 

protrusion  of  the  protoplasm  in  certain  animals. 

pupa,/w'/tf  (L.pupa,  a  doll),  a  stage  in  the  life  cycle  of  certain  insects. 
pyloric,  pilor'ik  (Gr.  pulorus,  a  gate-keeper),  of  or  pertaining  to  the  orifice 

between  the  stomach  and  the  intestine, 
pyrenoid,  ptre'noid  (Gr.   puren,    the  stone   of  fruit;    eidos,  form),  a  small 

colorless  mass  of  proteid. 

reaction,  re  ak'shon,  the  response  to  a  stimulus. 

recapitulation,  re  ka  pit  u  Id1  shon    (L.    re,   again;     capituluui,   a   head),  a 

theory  that  holds  that  the  individual  in  its  development  passes  through 

the  ancestral  stages  of  the  race  (see  p.  228). 
reduction,  reditk'shon   (L.  re,  back;   ductus,  led),  the  halving  of  the  number 

of  chromosomes  in  the  germ  nuclei  during  maturation. 
reflex,  re'Jleks  (L.  re,  back  ;  /exits,  bend),  bent  back  (see  p.  302). 
regeneration,  rejen  e  rd'shon  (L.  re,  again;  genero,  I  beget),  the  renewal  of 

a  portion  of  lost  or  removed  tissue. 
respiration,  res  pi  rd'shon  (L.  re,  back;   spiro,  I  breath),  the  absorption  of 

oxygen  and  excretion  of  carbon  dioxid. 
rostrum,  ros'trum  (L.  rostrtim,  a  beak),  a  beaklike  structure. 

saltation,  sal  ta' shon  (L.  saltatio,  dance),  an  abrupt  transition  or  change. 

saprophytic,  saprdfit'ik  (Gr.  sapros,  putrid;  phytos,  a  plant),  living  on  de- 
caying substances. 

scaphognathite,  skafognSthit  (Gr.  skaphe,  a  boat ;  gnathos,  a  jaw),  a  plate 
which  bales  the  water  out  of  the  branchial  chamber  of  the  crayfish. 

schizogony,  skizog'oni  (Gr.  schizo,  I  cleave),  reproduction  by  splitting. 

secretion,  sekre'shon  (L.  secretus,  separate),  the  process  of  separating  sub- 
stances by  glandular  activity. 

senescence,  se  nes'ens  (L.  senex,  old),  the  condition  of  growing  old. 

sensory,  sen1  so  ri  (L.  sensus,  sense),  capable  of  receiving  or  transmitting  im- 
pressions from  without. 

septum,  septum  (L.  septum,  a  partition),  a  wall  separating  two  cavities. 

seta,  se'td  (L.  seta,  a  thin,  stiff  hair),  bristles,  or  stiff  hairs. 

sexual,  sek'su  al  (L.  sexus,  sex),  of  or  pertaining  to  sex  ;  done  by  means  of 
the  two  sexes,  male  and  female. 

sinus,  si'nus  (L.  sinus,  hollow),  a  cavity  or  hollow  in  tissue. 

somatic,  somat'ik  (Gr.  soma,  body),  of  the  body;  applied  to  cells  that  do 
not  take  part  in  reproduction. 

somite,  so'mtt  (Gr.  soma,  body) ,  a  segment  of  an  articulated  body. 


GLOSSARY 


335 


spermatid,  sp'er  ma  fief  (Gr.  sperma,  seed),  a  cell  which  is  converted  into  a 

spermatozoon. 
spermatocyte,  sper'ma  to  sit  (Gr.  sperma,  seed),  a  stage  in  the  maturation  of 

a  spermatozoon. 
spermatogenesis,  sper'ma tojen'e sis  (Gr.  sperma,  seed;  gennao,  I  produce), 

the  origin  and  development  of  spermatozoa. 
spermatogonium,  sper'ma  togo'ni  um  (Gr.  sperma,  seed ;  gonos,  generation), 

a  male  germ  cell  just  before  maturation  takes  place. 
spermatozoon,  sper'ma  tozo'on    (Gr.  sperma,  seed  ;  zo on,  animal) ,  a  mature 

male  germ  cell, 
spireme,  spt'rem    (Gr.  speirao,  wind  round),  the  stage  in   mitotic  nuclear 

division  when  the  chromatin  is  in  the  form  of  a  thread. 
Spore,  spor  (Gr.  spora,  seed),  a  small  reproductive  body  of  certain  animals, 
statocyst,  stat'o sist  (Gr.  statos,  stationary;   kustis,*.  bladder),  the  organ  of 

equilibration  of  the  crayfish. 
statolith,  stat'o  lith  (Gr.  statos,  stationary;    lithos,  stone),  a  solid  particle 

within  a  statocyst. 

steapsin,  step' sin,  a  ferment  which  resolves  fats  into  fatty  acids  and  glyc- 
erin. 

sterile,  ster'il  (L.  sterilis,  barren),  not  reproducing  its  kind, 
sternum,  ster'num  (Gr.  sternon,  the  breast),  the  ventral  part  of  a  somite  of 

an  arthropod, 
stigma,  s tig' ma  (Gr.  stigma,  a  mark),  applied  to  spots  of  color,  or  to  small 

openings. 
stimulus,  stim'u  his    (L.  stimulus,  a  goad),  something  which  evokes  some 

functional  or  trophic  reaction  in  the  tissues  on  which  it  acts. 
symbiosis,  simbio'sis  (Gr.  sumbiosis,  a  living  together),  the  living  together 

of  two  different  species  of  organisms  (see  p.  297). 
systole,  sis' to  le  (Gr.  sustole,  a  drawing  together),  the  period  of  time  during 

which  a  rhymically  pulsating  vessel  is  contracted. 

telophase,  tel'ofdz  (Gr.  telos,  end),  the  last  phase  of  mitotic  cell  division, 

during  which  the  nuclei  are  re-formed, 
telson,  tel'son  (Gr.  telson,   the  end),  the  last  joint  of  the  abdomen  of  the 

crayfish. 

testis,  tes'tis  (L.  testis,  a  witness),  the  male  germ  gland, 
thermotropism,  th'er  mot'ropizm  (Gr.  thermot  heat  ;  trope,  turn),  reaction  to 

heat. 
thigmotropism,  thig mot 'ro pizm    (Gr.   thigma,   touch;    trope,  a  turning), 

reaction  to  contact. 
thorax,  tho'raks  (Gr.  thorax,  the  breast),  that  part  of  the   trunk  situated 

between  the  head  or  neck  and  the  abdomen. 


336 


GLOSSARY 


tissue,  tish'o  (L.  texere,  to  weave),  an  association  of  similar  cells  with  special 

functions  to  perform. 

trachea,  trd'ke  a  (Gr.  tracheia,  the  windpipe),  a  breathing  tube  of  an  insect, 
trichocyst,  trik'osist  (Gr.  thrix,  hair;   kustis,  a  bladder),  a  small  hair  like 

structure  of  certain  Protozoa  (see  p.  62). 
triploblastic,  trip  to  blas'tik  (Gr.  triploos,  threefold  ;  blastos,  a  germ),  having 

three  germ  layers. 

tropism,  tro'pizm  (Gr.  trope,  turn),  a  turning  caused  by  stimulus. 
typhlosole,  tif' Id  sol  (Gr.  tuphlos,  blind;  solen,  tube),  a  thick  fold  of  the 

intestine  of  certain  annelids. 

unicellular,  unisefular  (L.  unus,  one;  cellula,  a  cell),  consisting  of  a 
single  cell. 

uniramous,  u  ni  rd'mus  (L.  unus,  one  ;   ramus,  branch),  having  one  branch. 

uropod,  u'rdpod  (Gr.  oura,  tail ;  po^^,s,  foot),  the  last  appendage  of  the  cray- 
fish. 

uterus,  u'terus  (L.  uterus,  the  womb),  a  special  section  of  the  oviduct. 

vacuole,  vak'fi  ol  (L.  dim.  of  vacuus,   empty),  a  minute  vesicle  in   certain 

Protozoa. 
vagina,  vdji'na  (L.  vagina,  a  sheath),  the  passage  leading  from  the  uterus 

to  the  genital  orifice  in  certain  females. 
ventral,  ven'tral  (L.  venter,  the  belly),  of  or  pertaining  to  the  under  side  of 

the  body. 
Vitalism,  vital izm  (L.  vita,  life),  a  biological  doctrine  (seep.  24). 

zoology,  zo  oVoji  (Gr.  zoon,  animal  ;  logos,  discourse),  the  science  of  animals. 

zygote,  z?got  (Gr.  zugon,  a  yoke),  the  body  formed  by  the  conjugation  of 

two  gametes. 


INDEX 


An  asterisk  (*)  after  a  page  number  indicates  that  a  figure  of  the  object  named 
will  be  found  on  that  page  or  on  the  plate  facing  that  page.  Family,  generic; 
and  specific  names  of  animals  and  plants  are  printed  in  Italics. 


Abdomen:  Crayfish,  194*;  Honeybee, 
238-239,  244*. 

Aboral  surface,  59,  65. 

Absorption,  13,  46:  Ameba,  49;  Earth- 
worm, 171 ;  Paramecium,  67. 

Accretion,  growth  by,  13. 

Achromatic  substance,  32. 

Acoelomata,  5,  7,  in. 

Accelomate,  139. 

Adaptation,  282. 

Adaptive  power  of  animals,  16. 

Adjustor,  179. 

Afferent  gill  channels,  203. 

Afferent  nerve  fibers,  178. 

Air  sacs,  243*. 

Alchemist,  23. 

Alimentary  canal,  82,  89,  101,  in: 
Ascaris,  160,  161*;  Crayfish,  195*, 
200-201;  Earthworm,  168*,  169-170; 
Honeybee,  240,  241*,  242,  252;  Pla- 
naria,  154*,  155. 

Allolobophorafcetida,  182,  187,  188. 

Allolobophora  (Helodrilus)  longa,  164. 

Altman,  20. 

Alveolar  theory  of  protoplasmic  struc- 
ture, 20*. 

Ameba,  37*-s8,  59,  61,  64,  73,  81,  82,  101, 
127,  228;  anatomy,  38-41;  behavior, 
53-56;  growth,  50;  habitat,  38;  imi- 
tations, 45-49;  locomotion,  41-46, 
42*,  44*,  45*;  metabolism,  46-50; 
reproduction,  50,  51*,  52*,  53. 

Ameba binucleata,  52*. 

Ameba  verrucosa,  42,  43. 

Amebidce,  81. 

Amebulae,  52. 

Amitosis,  29,  32-33*. 

Amphiaster,  30*,  31. 

Amphiblastula,  149,  150* 

Anabolism,  13 


Anal  spot,  60*,  67. 
Anaphase,  30*,  31. 
Anatomy,  2,  270-271:  Ameba,  37*,  38- 

41;   Ascaris,  160-163,  161*;   Crayfish, 

195*,  200-211 ;  Earthworm,  i68*-i82; 

Euglena,   82-85,    83*;    Grantia,    145- 

148,  146*;   Honeybee,  234-250,  241*; 

Hydra,  119-125;   Paramecium,  59-64; 

Planaria,  154*-!  56. 
Ancestral  inheritance,  289. 
Ancon  sheep,  281-282. 
Andrena,  265*. 
Andrews,  E.  A.,  216. 
Animal  mind,  57,  305-306. 
Animalcules,  wheel,  5. 
Animals  compared  with  plants,  17-18. 
Annelida,  6,  7,  125,  190-192,  225,  227. 
Anopheles,  88,  91. 
Antenna:     Crayfish,     195*,     197,    216; 

Honeybee,  235*,  254. 
Antenna  cleaner,  236*,  237. 
Antenna  comb,  236*,  237. 
Antennary  artery,  195*,  202. 
Antennules,  195*,  197,  216. 
Anterior,  59. 
Anthozoa,  139. 
Antimere,  4,  5. 
Ants,  261,  264,  300. 
Anus,  4,  5,  6:   Ascaris,  160;    Crayfish, 

195*,     201,     216;     Earthworm,    167; 

Honeybee,  239,  240,  241*. 
Apida,  260,  261,  264. 
Apis,  260,  261. 
Apis  mettifica,  233-259,  234*. 
Apopyles,  145,  151*. 
Appendage:     Crayfish,    195-200,    196* 

209*;  Honeybee,  235-239,  236*. 
Arachnida,  225,  227,  228. 
Archenteron,  109*,  no 
Areola.  105 


337 


338 


INDEX 


Aristotle,  8,  267 "-268. 

Artemia,  32. 

Artery  :  Crayfish,  202-203 ;  Earthworm, 

171-174;  Honeybee,  242-243. 
Arthrobranchiae,  204. 
Arthropoda,  6,  7,  225-233. 
Ascaris  lumbricoides,  32,  160-163,  161*, 

165,  168,  171,  176,  277. 
Asexual  reproduction,  97. 
Assimilation,  13,  14,  46,  49,  171. 
Association  neuron,  179. 
Aster,  30*,  31. 
Astral  rays,  30*,  31. 
Attachment  cell,  255*. 
Autotomy,  220. 
Avoiding    reaction :    Euglena,    87,    88 ; 

Paramecium,  73-74*,  76. 

Bacteria,  29,  46,  66,  81,  259. 

Baer,  Karl  E.  von,  271,  272*. 

Balantidium  coli,  82. 

Balfour,  Francis  M.,  271,  272. 

Barnacle,  225. 

Barrier,  278. 

Basal  disk,  117,  118*,  123. 

Basipodite,  197,  198. 

Bateson,  W.,  290. 

Beaver,  300. 

Bee:   carpenter,  264;   leaf  cutting,  264; 

mining,    265-266;    social,    266;     soli- 
tary, 264-266. 
Bee  glue,  256. 
Bee  hive :  cleaning,  257 ;  guarding,  257- 

258;    number  of  individuals  in,  258; 

ventilating,  257. 
Bee  louse,  259. 
Bee  milk,  253. 
Bee  moth,  258-259. 
Bee  scouts,  258. 
Bee  tree,  258. 
Bees  and  flowers,  262-264. 
Bees  in  general,  260-266. 
Beetle,  301. 
Behavior:  Ameba,  53-58;  Crayfish,  221- 

224 ;  Earthworm,  186-189  '>  Euglena,  87  ; 

Hydra,  127-132;  Paramecium,  73-79. 
Beneden,  Van,  36. 
Bichat,  271. 

Bilateral  symmetry,  4,  5,  6,  7,  158,  186. 
Binary  division,   96,    112:    Ameba,   51*, 

52*:   Euglena,  83*,  87;   Hydra.  133*; 


Biobiotic  fauna,  277. 
Biochemist,  23. 
Biogenetic  law,  228-232. 
Biology,  i. 
Biophor,  288. 

Biramous  appendage,  196*. 
Birds  and  bees,  259. 
Blastocoel,  109*,  185. 
Blastoderm,  no:   Crayfish,  215;  Honey- 
bee, 252. 

Blastomere,  108,  109*. 
Blastophore,  184. 
Blastopore,  185*,  186. 
Blastula,    108,    109*,    no,    229:    Eartl 

worm,     185*;      Grantia,     149,     150*; 

Hydra,  118*,  137. 
Blood,  in:    Crayfish,  201-202;    Eartl 

worm,  171,  185*;  Honeybee,  242. 
Bloodvessels,   in:  Crayfish,  195*,  202; 

Earthworm,      172^175 ;      Honeyt 

241*,  242. 

Body  cavity  (see  Ccelom). 
Body  wall,  4,  5:   Ascaris,  162*; 

worm,  1 66*,  168. 
Bombus,  266. 
Bones,  101. 
Botany,  i. 
Bouton,  235*,  247*. 
Brachiopoda,  9. 
Brain  :  Crayfish,  195*,  205 ;  Eartln 

1 68*,    177*;     Honeybee,    244*; 

naria,  154*,  156. 
Branchial  chamber,  204. 
Branchial  filaments,  204. 
Branchiata,  225. 
Branchiocardiac  canals,  203. 
Branchiocardiac  grooves,  195. 
Branchiostegite,  195. 
Braula  caca  259. 

Breeding  habits,  Crayfish,  211-214 
Breeding  season,  Earthworm,  182. 
Brown,  34. 
Bryozoa,  5,  7. 
Buccal  pouch,  169. 
Budding,  15,  277:  Grantia,  148;  Hydra, 

118*,  133-134;  Protozoa,  80;  sponj 

152. 
Bumblebee,  262,  266;    guest,  297; 

orchid,  262,  263*. 
Butterfly,  279,  301. 

Calciferous  glands,  169-170 


INDEX 


339 


Cambarus  affinis,  193. 

Cambarus  pellucidus  lestii,  219. 

Cambarus  virilis,  193-224. 

Canal  system,  of  sponge,  151*. 

Capillary,  171,  175,  203. 

Carapace,  195. 

Carbohydrate,'  22,  170. 

Carbon,  in  protoplasm,  21. 

Carbon  dioxide,  50. 

Carcinus  manas,  299. 

Cardiac  stomach,  195*,  200,  201. 

Cattle  of  Paraguay,  281. 

Cell,  26-33,  27*  5  definition,  35 ;  divi- 
sion, 29-33*,  3°*5  form,  29;  mor- 
phology, 26-29 ;  number,  29 ;  origin, 
33.  34*J  physiology,  26-29;  size,  29; 
unit,  12,  26;  wall,  28,  35. 

Cell  theory,  34-36. 

Cellulose,  93. 

Cement  glands,  213. 

Centipede,  6,  225,  226*,  227. 

Central  nervous  system  :  Crayfish,  195*, 
205;  Earthworm,  177-178;  Honeybee, 
241*,  244-245. 

Centrosome,  27*,  28,  30,  31,  104. 

Centrosphere,  27*,  28,  30. 

Cephalothorax,  194,  195. 

Ceratina  dupla,  264. 

Cervical  groove,  195. 

Cestoda,  158. 

Chaetopoda,  190. 

Chela,  195*,  198. 

Cheliped,  195*,  196,  198. 

Chemical  composition  of  living  organisms, 
11-12. 

Chemotropism,  53:  Ameba,  55*;  Cray- 
fish, 222-223;  Earthworm,  187-188; 
Hydra,  132;  Paramecium,  75*,  78. 

Chilopoda,  227. 

Chitin,  194,  234. 

Chlamydomonas,  92*,  93,  94,  96. 

Chlorogogen  cells,  166*,  169. 

Chlorophyll,  17,  1 8,  84,  85,  88,  93,  94,  97. 

Chloroplast,  28. 

Choanocyte,  146*,  147,  152. 

Chorda,  6. 

Chordata,  5. 

Chorion,  251. 

Chromatin,  27*,  30,  31,  32 ;  continuity  of, 
33- 

Chromatophore,  81,  83*,  84,  85,  93. 

Chromosome,  30*,  69,  103,  104,  105,  106 ; 


bearer  of  hereditary  qualities,  32;  in 
germ  plasm  theory,  288;  in  Hydra, 
134,  136;  number  in  animals,  32;  re- 
duction in  number,  103*,  104*,  105*, 
107 ;  union  in  fertilization,  105*,  107. 

Chromotropism,  54,  56. 

Chrysalis,  254. 

Chyle,  253- 

Chyme,  240. 

Cilia,  28,  154,  155;  Paramecium,  61-62, 
64,65,66,76*,  77,80,81. 

Ciliate,  82. 

Circulation,  46,  49,  in  :  Crayfish,  203- 
204;  Earthworm,  171,  174-175;  Honey- 
bee, 242-243. 

Circumoesophageal  connective,  205. 

Circumpharyngeal  connective,  177. 

Cirrus,  156. 

Clam,  6. 

Class,  3. 

Classification,  3-7;  Honeybee,  260-261. 

Claw,  237*. 

Cleavage,  107-110,  109*;  discoidal,  108*; 
equal,  108*;  partial,  108;  superficial, 
108*;  total,  108*;  unequal,  108*. 

Cleavage  cavity,  no. 

Clitellum,  165,  182,  183. 

Cloaca,  145,  151*. 

Clypeus,  235*. 

Cnidoblast,  120,  121*,  123,  124. 

Cnidocil,  120,  121*,  123. 

Coagulation,  22,  202. 

Cocoon:  Earthworm,  183*;  Honeybee, 
253;  Planaria,  156. 

Coelenterata,  5,  7,  116,  119,  139-143,  158, 
228,  229. 

Ccelenteron,  109*,  139. 

Coelom,  4,  5,  6,  in,  138,  158,  159: 
Ascaris,  160,  162,  163;  Crayfish,  200; 
Earthworm,  166*,  168,  186;  Honey- 
bee, 239;  Leech,  190;  Planaria,  158. 

Coelomata,  5,  7,  in. 

Coelomic  fluid,  171. 

Cohn,  35. 

Colon,  240. 

Collar  cells,  146*,  147,  148. 

Colloid,  22. 

Comb  jelly,  5. 

Commensalism,  297. 

Community  life  of  bees,  266. 

Comparative  anatomy,  history  of,  270- 
271. 


340 


INDEX 


Conduction,  178. 

Conductivity,  56. 

Conjugation,  112,  113,  114:  of  chromo- 
somes, 103 ;  Pandorina,  94 ;  Para- 
mecium,  67,  68-71,  69*,  70*,  72,  73. 

Connective  tissue,  101*,  in. 

Conoid  hairs,  246*,  248. 

Consciousness,  57. 

Continuity  of  germ  plasm,  99*,  288- 
289. 

Contractile  theory  of  locomotion  of 
Ameba,  42-45. 

Contractile  vacuole:  Ameba,  37*,  39-41, 
40*,  50 ;  Chlamydomonas,  93 ;  Eu- 
glena,  81,  82,  83*;  Paramecium,  60, 
63-64*,  67,  68;  Volwx,  97. 

Cope,  E.  D.,  270. 

Copulation:  Crayfish,  2i2*-2i3;  Earth- 
worm, 182-183*;  Honeybee,  250-251. 

Copulatory  organs  of  Honeybee,  239, 
249*. 

Coral,  5,  139,  142-143. 

Corpuscles,  171,  202. 

Coxa,  236*. 

Coxopodite,  197,  198. 

Crab,  6,  143*,  225. 

Crayfish,  193-224,  276,  277,  306;  adap- 
tations, 282 ;  anatomy,  194-211,  195*; 
appendages,  i95*-2oo,  196*;  autotomy, 
220-221;  behavior,  221-224;  breeding 
habits,  211,  212*,  213*,  214*;  circula- 
tion, 203-204;  digestive  system,  195*, 
200-201;  embryology,  214,  215*,  216, 
217*;  excretory  system,  205;  muscu- 
lar system^  194*,  209*;  nervous  sys- 
tem, 195*,  205;  regeneration,  219*- 
220;  reproductive  system,  209,  210*, 
2ii*;  respiratory  system,  204;  sense 
organs,  205-208 ;  vascular  system,  201- 
204. 

Crop,  1 68*,  169,  170. 

Cross  pollination,  246,  262,  263*,  264. 

Crustacea,  125,  219,  225*,  226,  227,  229, 
232. 

Crystal,  28. 

Crystalloid,  22. 

Ctenophora,  5,  7. 

Culex,  88,  92. 

Cuticle,  101 :  Ascaris,  163 ;  Crayfish, 
194;  Earthworm,  166*,  168;  Euglena, 
82,  83*;  Honeybee,  234;  Hydra,  119; 
Paramecium,  59,  60*,  61. 


Cuvier,  270*. 

Cyclops,  125,  225*,  232. 

Cyclosis,  60*,  66. 

Cypripedium,  263*. 

Cypris,  299*. 

Cyst,  96:  Ameba,  52;   Euglena,  83*,  86 

87. 

Cytopharynx,  59,  66*. 
Cytoplasm,  20*,  26,  27,  28. 

Darwin,  Charles,  8,  262,  273-274*,  278, 
293,  294,  295,  296. 

Death,  98. 

Deer,  300. 

Dellinger,  O.  P.,  43,  45. 

Dermal  epithelium,  146,  151*. 

Dermal  pores,  152. 

Determinant,  288. 

Devilfish,  6. 

Diaphragm,  242,  244*. 

Diastase,  170. 

Diatom,  46. 

Didinium,  63*. 

Digestion,  13,  14,  46;  extracellular,  127; 
intracellular,  127:  Ameba,  49;  Cray- 
fish, 201;  Earthworm,  170;  Grantia, 
148;  Honeybee,  240;  Hydra,  127; 
Paramecium,  67. 

Digestive  gland,  201. 

Digestive  system:  Ascaris,  160-161*; 
Crayfish,  195*,  200-201 ;  Earthworm, 
168*,  169-170;  Honeybee,  240,  241*, 
242 :  Planaria,  154*,  155,  158. 

Dioecious,  209. 

Diploblastic,  4,  5,  7,  no,  159. 

Diplopoda,  227. 

Direct  cell  division,  29,  32-33*. 

Discontinuous  distribution,  278-279. 

Discontinuous  variation,  281. 

Diseases  of  bees,  259. 

Dispersion,  277-278. 

Dissimilation,  46,  49-50,  67. 

Distal,  117. 

Distribution  of  animals:  in  space,  275- 
279;  in  time,  279-280*. 

Dorsal,  59;  abdominal  artery,  195*, 
203;  bloodvessel,  168,  173,  241*,  242; 
pore,  1 68. 

Drone  honeybee,   231*,   234,   254;    cell, 

255*. 

Dujardin,  19. 
Dysentery,  amebic,  82 ;  of  bees,  259, 


INDEX 


341 


Earthworm,  in,  115,  164-190,  166*, 
168*,  194,  219,  275,  276,  277;  anatomy, 
164-185,  168*;  behavior,  186-189; 
digestive  system,  168*,  169-170;  em- 
bryology, 1 85*-!  86;  excretory  organs, 
1 66*,  175-176;  grafting,  190* ;  nervous 
system,  176-180,  177*;  nutrition,  170- 
171;  regeneration,  i89*-i9o;  repro- 
duction, 180-185,  181*;  respiration, 

I75'. 
Ecdysis,  216. 

Echinodermata,  6,  7. 

Ecology,  2. 

Ectoderm,  no,  114,  115;  organs  arising 
from,  in:  Ascaris,  162;  Earthworm, 
186;  Honeybee,  252;  Hydra,  119-124; 
Planaria,  154. 

Ectoplasm:  Ameba,  38;  Paramecium, 
61,  81. 

Ectosarc:  Ameba,  37*,  38;  Euglena,  82; 
Paramecium,  59,  60*. 

Effector,  179. 

Efferent  gill  channels,  203. 

Efferent  nerve  fibers,  178,  179*. 

Egestion,  46;  Ameba,  49;  Hydra,  127. 

Egg,  15,  96,  98,  99,  104,  106,  107,  113; 
holoblastic,  108,  109*;  meroblastic, 
108*:  Ascaris,  161 ;  Crayfish,  211, 
214,  215*;  Earthworm,  185*;  Grantia, 
149;  Honeybee,  250,  251,  252;  Hy- 
dra, 118*,  135-137,  136*;  Planaria, 
156. 

Egg-laying:  Crayfish,  2 13*-2 14;  Honey- 
bee, 251. 

Egg  sac,  181*. 

Eimer,  295. 

Ejaculatory  duct,  161,  248,  249*. 

Elasticity,  of  Euglena,  83*,  85. 

Electrotropism,  53:  Ameba,  56;  Hydra, 
131-132;  Paramecium,  77-78. 

Elementary  species,  296. 

Embryogeny,  108. 

Embryology,  2,  107-115,  291;  historical, 
271-272:  Crayfish,  214-216,  215*, 
217*;  Earthworm,  i8s*-i86;  Grantia, 
149;  Honeybee,  251-252*;  Hydra, 
118*,  137;  Planaria,  156-157*; 
Sponges,  150*,  152-153. 

Encystment,  112:  Ameba,  52;  Euglena, 
83*,  85-86. 

Endoplasm,  81 :  Ameba,  38;  Parame- 
cium, 60. 


Endopodite,  194*,  195,  197,  198,  199. 

Endosarc:  Ameba,  37*,  38;  Euglena, 
82,  83* ;  Paramecium,  59,  60*. 

Energy,  of  metabolism,  13,  49. 

Entameba  histolytica,  82. 

Enteron,  186. 

Entoderm,  no,  114,  115;  organs  arising 
from,  in:  Earthworm,  186;  Honey- 
bee, 252;  Hydra,  118*,  119,  124-125; 
Planaria,  154. 

Entodermal  plates,  216,  217*. 

Entomesoderm,  252. 

Entomostraca,  225*,  226. 

Environment,  73,  282. 

Enzyme,  25,  127,  170. 

Epidermis,  in,  167*,  168. 

Epigenesis,  272. 

Epimeron,  194*,  195. 

Epipharynx,  235*. 

Epipodite,  196*,  197,  205. 

Epithelial  tissue,  ioo*-ioi. 

Epitheliomuscular  cells,  119,  123,  125. 

Epithelium,  in. 

Equatorial  plate,  30*,  31. 

Equilibration,  208,  221-222. 

Equus,  292—293. 

Eudorina  elegans,  96. 

Euglena,  62,  81,  82-88,  83*,  276,  284; 
anatomy,  82-85,  83*;  behavior,  87; 
encystment,  83*,  85 ;  locomotion,  85, 
86*;  nutrition,  85;  reproduction,  83*, 
87.  ^ 

Euglenida,  81. 

Euglenidce,  81. 

Evening  primrose,  295. 

Evolution,  291-297;  arguments  for, 
291-294;  of  horse,  292-293;  of  sex, 
92 ;  theories,  293-297. 

Evolutionary  zoology,  2;  historical, 
273-274. 

Excretion,  14,  159;  Ameba,  50;  Earth- 
worm, 176;  Grantia,  148;  Parame- 
cium, 64,  67. 

Excretory  system,  in:  Ascaris,  161*; 
Crayfish,  195*,  205;  Earthworm, 
166*,  175-176;  Honeybee,  242;  Pla- 
naria, 155,  156*. 

Excurrent,  canal,  151*;  pore,  144, 
151*. 

Exopodite,  194*,  195,  197, 198, 199- 

Exoskeleton:  Crayfish,  194;  Honeybee, 
234- 


342 


INDEX 


Extensor  muscle,  194*,  209. 

Extinction  of  animals,  295. 

Eye:  Crayfish,  195*,  205-208;  Honey- 
bee, 235*,  245-246,  254. 

Eye  brush,  236*,  237. 

Eye  spot,  81,  83*,  84,  94,  97;  Planaria, 
153*,  154- 


Factors  of  habitat,  275-276. 

Faeces,  13,  14,  155,  242. 

Family,  3. 

Fatigue,  a  stimulus,  16. 

Fat,  21,  22,  170. 

Fauna :  of  islands,  285-286 ;  tabulated, 
277. 

Female,  98. 

Femur,  236*. 

Ferment,  25,  170. 

Fertilization,  104,  105*,  107 ;  chromo- 
somes in,  107;  nuclei  in,  106:  Ascaris, 
161 ;  Crayfish,  214;  Earthworm,  185  ; 
Eudorina,  96;  Grantia,  149;  Honey- 
bee, 251;  Hydra,  114,  137;  Parame- 
cium,  67,  69*,  71,  112;  Planaria,  156; 
Plasmodium,  88*,  91,  113;  Volwx,  98, 
114. 

Fever :  estivo-autumnal,  89 ;  malarial, 
88,  89,  91-92 ;  pernicious,  89 ;  quar- 
tan, 89;  tertian,  89;  yellow,  80. 

Fission,  15:  Ameba,  51*,  52;  Euglena, 
83*,  87;  Hydra,  133*;  Paramecium, 
67*-68;  Planaria,  156. 

Flagellata,  82. 

Flagellum,  62*,  80,  81,  83*,  85,  86,  87,  93, 
94,  96,  97,  124,  147. 

Flame  cell,  155,  156*. 

Flat  worms,  5,  153-160;  classification, 
158;  contrasted  with  Hydra,  159. 

Flea,  298. 

Flexor  muscle,  194*,  209. 

Flight,  238. 

Flowers  and  bees,  262,  263*,  264. 

Fluctuating  variation,  281. 

Fol,  36. 

Foliaceous  appendage,  196*. 

Food:  Ameba,  46;  Crayfish,  201 ;  Earth- 
worm, 170;  Euglena,  85;  Grantia, 
148;  Honeybee,  240,  256;  Hydra,  125; 
Paramecium,  66;  Planaria,  155. 

Food  vacuole,  148:  Ameba,  37*,  47*; 
Paramecium,  60*.  66*-67. 


Foot:    Honeybee,    237*;    Horse,    293*; 

Hydra,  117,  118. 
Forel,  305. 

Form,  of  organisms,  n. 
Fossil,  2,  279;  number,  5,  6. 
Foul  brood,  259. 
Fusion  nucleus,  71,  105*. 

Galapagos  Islands,  278. 

Galen,  Claudius,  268. 

Galleria  mellonella,  258-259. 

Galton,  Francis,  289. 

Gamete,  90,  93,  94,  95*,  96,  113. 

Garnet  ogenesis,  113. 

Ganglion,  205. 

Gas,  in  protoplasm,  21. 

Gastraea,  229. 

Gastral  cavity,  151*. 

Gastral  epithelium,  146,  151*. 

Gastric  mill,  200. 

Gastrolith,  200-201. 

Gastrovascular  cavity,  5,  118*,  119,  127, 

139- 
Gastrula,  108,  109*,  no,  in,  137,  185*, 

1 86,  229. 

Gastrulation,  109*,  no. 
Gemmule,  152. 
Genital  aperture,  161,  210,  211;    cloaca, 

156;   pore,  153*,  154,  161*. 
Genus,  3. 

Geobiotic  fauna,  277. 
Geographical  distribution,  293. 
Geographical  isolation,  285-286. 
Geological  periods,  279,  280. 
Geotropism,  54:  Paramecium,  77*,  78. 
Germ  band,  Honeybee,  252. 
Germ  cells,  98,  102-107,  114,  115. 
Germ  layers,   108,   109*,   no,   in,  154, 

159,  1 86. 

Germ  plasm,  98-99*,  102. 
Germ  plasm  theory,  99*,  288-289. 
Germinal  vesicle,  105*,  1 06. 
Geryonia,  142. 
Gesner,  Conrad,  269. 
Giant  fibers,  178,  179*. 
Gills,  195,  204. 
Gizzard,  168*,  169. 
Gland  cells,  124*,  169. 
Goodsir,  36. 
Grafting,  24:  Earthworm,  190*;  Hydra, 

137*,  138. 
Grantia  ciliata,  144-149,  277;    anatomy 


INDEX 


343 


145,  146*,  147*,  148;  embryology, 
149,  150*;  nutrition,  148;  reproduc- 
tion, 148-149. 

Granular  theory  of  protoplasmic  struc- 
ture, 20. 

Gravity,  54,  221-222. 

Green  gland,  195*,  205. 

Growth,  13-15;  by  accretion,  13;  by 
intussusception,  13:  Ameba,  50; 
Paramecium,  67. 

Gullet:  Euglena,  82,  83*,  84;  Parame- 
cium, 59,  60*,  67. 

Gymnameba,  81. 

Gynandromorph,  260,  262. 

Habit  formation,  Crayfish,  223-224. 

Habitat,  275-277,  284. 

Haeckel,  229. 

Haematochrome,  84. 

Haemoglobin,  90,  171. 

Hair  of  Honeybee,  236*. 

Halictus,  265*,  266. 

Haller,  Albrecht,  272-273. 

Halobiotic  fauna,  277. 

Harvey,  William,  271*,  272. 

Hatching:  Crayfish,  216,  217*;  Honey- 
bee, 253 ;  Hydra,  137. 

Head,  Honeybee,  235*-236. 

Hearing,  Honeybee,  247-248. 

Heart :  Crayfish,  195*,  202  ;  Earthworm, 
172*,  173;  Honeybee,  241*,  242. 

Hemosporidia,  82. 

Hepatic  duct,  201,  202-203. 

Heredity,  284-285,  287-290;  acquired 
characters,  287-288;  and  evolution, 
275-297;  in  Paramecium,  79;  meth- 
ods of  study,  285. 

Hermaphrodite,  149,  156. 

Hermit  crab,  143*,  297. 

Heron,  300. 

Hertwig,  36. 

Heteromorphosis,  189,  219*,  220. 

Hibernation,  276. 

Highways  of  dispersal,  278. 

Hirudinea,  190. 

Histology,  2;   historical,  27  T 

Hofer,  49. 

Hofmeister,  35. 

Holmes,  S.  J.,  224. 

Holoblastic  egg,  108,  109*. 

Holophytic  nutrition,  85. 

Holotrichida,  81. 


Holozoic  nutrition,  85. 

Homo,  3. 

Homologous  structures,  291,  292*. 

Honey,  256-257;  cells,  256;  comb,  254, 
255*,  256;  flavor,  257;  sac,  240,  241*, 
257- 

Honeybee,  15,  233-259,  275,  277,  284; 
activities  of  workers,  254-258;  adapta- 
tions, 282;  anatomy,  234-250',  241*; 
diseases,  259;  drone,  234*;  embry- 
ology, 251-252*;  enemies,  258-259; 
instincts,  303-304 ;  metamorphosis, 
252*,  253*,  254;  queen,  233,  234*; 
reproduction,  248*-25i,  249*;  worker, 

234*- 

Hooke,  34. 

Horse,  ancestry,  3 ;  evolution,  202-293*. 

Huggins,  224. 

Hunger,  16. 

Huxley,  19,  193,  262,  270. 

Hydra  dioecia,  116. 

Hydra  fusca,  in,  116-139,  118*,  128*, 
129*,  141,  155,  160,  171,  219,  228,  277, 
278,  284,  297;  anatomy,  117-125, 
118*;  behavior,  127,  128*,  129*,  132; 
contrasted  with  Planaria,  159;  nutri- 
tion, 125-127;  regeneration,  i37*-i39J 
reproduction,  i33*-i37. 

Hydra  grisea,  116. 

Hydra  viridis,  116,  117,  127,  143. 

Hydrogen,  in  protoplasm,  21. 

Hydrozoa,  139,  141. 

Hymenoptera,  260,  261. 

Hypostome,  117,  118*. 

Id,  288. 

Idioplasm,  288. 

Ileum,  240,  241*. 

Imago,  234*,  252. 

Imitations,  of  Ameba,  45-46,  47-49. 

Incurrent  canal,  145,  151*,  152. 

Incurrent  pore,  145*. 

Indirect  cell  division,  29-32,  30*. 

Individual,  3,  15,  71,  151. 

Inferolateral  nerve,  205. 

Infusoria,  80,  81,  101. 

Ingestion:  Ameba,  46-49,  47*,  48*;  Cray- 
fish, 201 ;  Earthworm,  170;  Grantia, 
148;  Hydra,  125-126;  Paramecium, 
66*. 

Inheritance :  acquired  characters,  287- 
288;  ancestral,  289;  germ  plasm  in. 


344 


INDEX 


288;   Mendel's  law  of,  289-290*;   of 

instincts,  305. 

Insecta,  6,  225,  227,  233,  261. 
Instinct,  303-305 ;    Honeybee,  303-304. 
Interbreeding,  286. 
Intercellular  digestion,  155. 
Interstitial  cells,  118*,  119,  121,  123,  134. 
Interzonal  fibers,  30*,  31. 
Intestine:    Ascaris,  160,  161*,  162,   163; 

Crayfish,  195*,  201 ;  Earthworm,  168*, 

169;  Honeybee,  240,  241*,  252. 
Intestino-tegumentary  blood  vessel,  172*, 

173- 

Intracellular  digestion,  148,  155. 

Intussusception,  a  method  of  growth,  50. 

Invertebrata,  5. 

Irritability,  16,  17,  56. 

Isolation  :  geographical,  285-286 ;  physi- 
ological, 286. 

Jellyfish,  5,  139. 

Jennings,  H.  S.,  38,  42,  46,  53,  57. 

Karyoplasm,  26. 
Karyosome,  27*. 
Katabolism,   13,   14*,  40-50. 
King  crab,  225,  228. 
Kolliker,  35. 
Kreidl,  208. 
Kropotkin,  301. 

Labial  palpus,  235*. 
Labium,  235*. 
Labrum,  216,  235*. 
Lamarck,  273,  287. 

Larva:  Crayfish,  216,  217*,  218;  Honey- 
bee, 252,  253*. 

Lateral  blood  vessel,  172*,  173. 
Lateral  line,  160,  162*. 
Lateral  nerve,  205. 
Laveran,  88. 
Leech,  190,  191*. 
Leg:     Crayfish,    196*,    198;    Honeybee, 

236*-238. 

Lichen,  297. 

Liebig,  24. 

Life,    origin,    8-10;     phenomena,    8-25. 

Life  cycle,  14-15,  80;  Paramecium,  71-73. 

Ligula,  235*. 

Limnobiotic  fauna,  277. 

Linin,  27*. 

Linnaeus,  Carl,  4,  269*,  270. 


Lips,  of  Ascaris,  160. 

Liriope,  142. 

Liver,  201. 

Liver  fluke,  158. 

Lobster,  229*,  230,  277. 

Locomotion,  17,  18:   Ameba,  41-46,  42* 

44*,  45*,  Crayfish,  221;    Euglena,  85; 

Hydra,    i29*-i3o;     Paramecium,    64, 

65*,  66;  Planaria,  154. 
Loeb,  J.,  304. 
Lumbricus  terrestris,  164-190. 

Macrogamete,  89*,  90,  96,  98. 

Macrogametocyte,  89*,  90,  91. 

Macromere,  185*. 

Macronucleus,  60*,  67,  68-71,  81. 

Malacostraca,  226,  229. 

Malaria,  88,  89,  91,  92. 

Male,  98. 

Malpighi,  242,  271. 

Malpighian  tubule,  241*,  242,  253. 

Mammalia,  3. 

Mandible:  Crayfish,  195*,  196,  197,  216; 

Honeybee,  235*. 
Mastigophora,  80,  81,  82. 
Maturation,  103,  105*. 
Maturity,  14,  71,  72. 
Mauchamp  sheep,  281. 
Maxilla,  196,  197,  235*. 
Maxillary  palpus,  235*. 
Maxilliped,  195*,  196,  197,  198. 
Mechanistic  theory  of  life,  23-25. 
Medusa,  139,  140*,  141*,  142. 
Megachile  acuta,  264. 
Melanion,  90. 
Membrane!!,  62,  66*. 
Mendel.  Gregor,  274. 
Mendel's  law  of  inheritance,   289-290*. 
Mentum,  235. 
Meroblastic  egg,  108*. 
Merozoite,  89*,  90,  113. 
Mesoblastic  bands,  185*. 
Mesoderm,  no,  115  ;  organs  arising  from, 

in:     Earthworm,     186;     Honeybee, 

252;  Planaria,  154. 
Mesoglea,  118*,  119,  125. 
Mesohippus,  292. 
Mesomere,  185*. 
Mesothorax,  236. 
Metabolism,  13,  14*,  17,  46,  72. 
Metagenesis,  141-142. 
Metamere,  4,  165. 


INDEX 


345 


Metamorphosis  of  Honeybee,  252,  253*, 

254- 

Metaphase  of  mitosis,  30*,  31. 
Metaplasm,  26,  27*. 
Metathorax,  236. 
Metazoa,  5,  29,  33,  71,  100,  103,  in,  112, 

22Q. 

Microgamete,  89*,  90,  91,  96,  98. 
Microgametocyte,  89*,  90,  91. 
Micromere,  185*. 

Micronucleus,  60*,  67,  68-71,  81. 
Micro-organism,  10. 
Migration,  276,  285. 
Millipede,  225,  227*. 
Mimosa  pudica,  18. 
Mind,  305. 
Mining  bee,  265*. 
Mite,  225,  228. 
Mitosis,  29,  30*,  31-32,  67. 
Mole,  277. 
Mollusca,  6,  7. 

Molt:    Crayfish,  216,  217,  218;   Honey- 
bee, 253. 
Monoecious,  149. 
Morphology,  2. 
Morula,  108,  109*,  no. 
Mosquito,  88,  89,  90,  91,  92. 
Moss  animals,  5. 
Motor  nerve  cell,  178,  179*. 
Motor  neuron,  179*. 
Mouth,  4:  Ascaris,  160;   Crayfish,  195*, 

200,  216;  Earthworm,  167,  168*,  186; 

Euglena,  82,  83*;    Hydra,   117,   118*; 

Paramecium,  59,  60*,  68,  81 ;  Planar la, 

154*,  155- 

Mouth  parts  of  honeybee,  235*-236,  254. 
Mucous  gland,  248. 
Miiller,  J.,  246,  271,  272,  273*. 
Muscle,  168,  169;   fibers,  101*,  119. 
Muscular   system,    in,    159:     Crayfish, 

209 ;    Honeybee,    239-240 ; .  Planaria, 

155- 

Muscular  tissue,  101*. 
Mutant,  295. 

Mutation,  281 ;   theory,  274,  295-297. 
Myocyte,  148. 
Myoneme,  101. 
Myriopoda,  225,  227. 
My  sis,  229*,  230*,  231*,  232. 

Nageli,  35. 

Natural  selection,  293-294. 


Nauplius,  216,  226,  230*,  231,  232,  299*. 

Nectar,  235-236,  257. 

Nemathelminthes,  5,  7,  163. 

Nematocyst,  5,  120*,  121*,  i22*-i23,  125, 
139- 

Nephridiopore,  166*,  167. 

Nephridium,  166*,  173,  174,  175,  176. 

Nephrostome,  175-176*. 

Nereis,  190,  191*. 

Neuron,  179*;  theory,  178. 

Nervous  system,  17,  in:  Ascaris,  161 ; 
Crayfish,  195*,  205 ;  Earthworm,  176- 
180*,  177*,  179*;  Honeybee,  244^245, 
253;  Hydra,  125;  Planaria,  154*, 
IS5-I56. 

Nervous  tissue,  101-102. 

Niata  oxen,  281. 

Nitrogen,  in  protoplasm,  21. 

Nucleolus,  27*,  32,  84. 

Nucleoplasm,  27*. 

Nucleus,  26,  27*,  28,  30,  31,  35 ;  division, 
29-33,  3°*;  in  fertilization,  105*,  106  : 
Ameba,  37*,  38,  39;  Euglena,  83*,  84; 
Paramecium,  60*,  68,  69,  70,  71. 

Nutrition,  holophytic,  85  ;  holozoic,  85  ; 
saprophytic,  85  :  Crayfish,  201 ;  Earth- 
worm, 170-171 ;  Euglena,  85;  Grantia, 
148;  Paramecium,  66-67. 

Obelia,  139,  140*,  142. 

Ocelli,  235*,  245. 

(Enothera  lamarckiana,  295. 

(Esophagus:  Crayfish,  195*,  200;  Earth- 
worm, 1 68*,  169,  170;  Honeybee, 
240,  241*,  253. 

Oligochaeta,  190,  192. 

Old  age,  14,  71,  72. 

Ommatidium:  Crayfish,  206*,  207*; 
Honeybee,  245. 

Ontogeny,  108,  229. 

Onychophora,  225,  226,  227. 

Oocyte,  104*,  105*,  106. 

Oogenesis,  103,  io4*-io6:  Crayfish,  211; 
Earthworm,  184-185;  Grantia,  149; 
Honeybee,  250;  Hydra,  118*,  135-136*. 

Oogonia,  104*. 

Ookinet,  89*,  91. 

Ophthalmic  artery,  195*,  202. 

Optic  lobe,  216,  217*,  244*. 

Optic  nerve,  207. 

Optimum,  76,  88. 

Oral  groove;  59,  60*,  65,  81. 


346 


INDEX 


Order,  3. 

Organ,  in. 

Organic  evolution,  228. 

Organism,  characteristics,  10-16. 

Organization  of  bodies,  12-13. 

Organogeny,  108,  in. 

Origin  of  flowers,  246. 

Orohippus,  292. 

Orthogenesis,  294-295. 

OsciUaria,  46,  48*. 

Osculum,  144, 145*,  149,  151*. 

Osmosis,  171. 

Ostia,  145, 151*,  152,  202,  242. 

Ovary:  Ascaris,  161*;  Crayfish,  211*; 
Earthworm,  180,  181*,  184,  185; 
Honeybee,  249*,  250 ;  Hydra,  118*,  119, 
135;  Planaria,  154*,  156. 

Oviduct:  Crayfish,  211*;  Earthworm, 
167,  180,  181*;  Honeybee,  249*,  250; 
Planaria,  154*,  156. 

Ovum,  106. 

Owen,  Richard,  270. 

Oxidation,  in  metabolism,  13,  14. 

Oxygen,  in  living  matter,  1 1 ;  in  metabo- 
lism, 13,  14;  in  protoplasm,  21. 

Oyster,  283. 

Palcemon,  219*,  220. 

Paleontology,  2,  279-280,  292. 

Paleozoology,  2. 

Pandorinamorum,  94,  95*,  96,  no,  113. 

Paramecidce,  Si. 

Paramecium  candatum,  15,  59-79,  81,  82, 
87,  108,  127,  275,  277,  302;  anatomy, 
59-64,  60*;  behavior,  73-79;  hered- 
ity, 79;  life  cycle,  71-73;  locomotion, 
64,  65*,  66;  nutrition,  66-67;  repro- 
duction, 67*-7i,  69*. 

Paramylum,  83*,  84. 

Parasitic  Protozoa,  80,  82. 

Parasitism,  298-300. 

Parazoa,  5. 

Pareiopod,  195*,  198. 

Parietal  blood  vessel,  172*,  173. 

Parthenogonidia,  97*. 

Pasteur,  9. 

Pea,  hybrid,  289-290. 

Pellicle,  59,  60*,  61,  83*. 

PencRus,  230*,  231*,  232. 

Penial  setae,  160. 

Penis,  154*,  156,  249*. 

Perception,  178. 


Pericardial  sinus,  202,  242. 

Peripatus,  225,  226*,  227. 

Peripheral  nervous  system,  177-178. 

Peristaltic  contraction,  175. 

Peristome,  59,  60*. 

Peritoneum,  168. 

Perivisceral  sinus,  203. 

Pharynx,    81,    in,    159:    Ascaris,    n 
161*;    Earthworm,    168*,     169,     170; 
Planaria,  154*,  155. 

Photosynthesis,  18*,  28,  84. 

Phototropism,  53:  Ameba,  56*;  Cray- 
fish, 223;  Earthworm,  188;  Euglcna, 
86*,  87*;  Hydra,  131. 

Phyla,  of  animals,  5-7*;  characteristics 
separating,  4. 

Phytogeny,  229. 

Physalia,  142,  143*. 

Physicochemical  theory  of  life,  23-25. 

Physiological  continuity  between  cells, 
97*. 

Physiological  isolation,  286. 

Physiological  states,  78,  189. 

Physiology,  2;  historical,  272-273. 

Pigeons,  293-294*. 

Pincher,  195*,  196,  198. 

Pithecanthropus,  3. 

Planaria  lactea,  157. 

Planaria  maculata,  i53*-i57*,  165,  171 
176,  277;  anatomy,  153-156,  154*; 
regeneration,  157*. 

Planaria  polychroa,  153*. 

Plants,  compared  with  animals,  17-18. 

Plasmodiumfalciparum,  89. 

Plasmodium  malaria,  89. 

Plasmodium    vivax,     82,     88*-92,     277; 
discovery  of,  88 ;  life  history,  89*-gi 
reproduction,  90-91 ;   transmission 
mosquitoes,  88,  89,  90,  91,  92. 

Plastid,  27*,  28. 

Platyhelminthes,  5,  7,  153. 

Pleopod,  195*,  198,  190 

Pleurobranchia,  204. 

Pleuron,  194*,  195. 

Pliny,  268. 

Pliohippus,  292. 

Podobranchia,  204. 

Podocoryne  carnea,  143. 

Poison  gland,  Honeybee,  239*. 

Polar  bodies,  68,  91,  105*,  106,  113,  ii< 
115,  136,  149. 

Pollen,   basket,    236*,    237-238;    bn 


INDEX 


347 


236*;     comb,    236*,    238;     gathering, 

236*,  256. 

Polychaeta,  190,  192. 
Polyp,  5,  139. 
Porifera,  5,  7,  149. 
Porocyte,  146,  148. 
Porospora  gigantea,  80. 
Portuguese  Man-of-War,  142,  143*. 
Posterior,  59. 
Prairie  dog,  30x5. 
Preformation  theory,  271-272. 
Primates,  3. 
Primitive  streak,  216. 
Proboscis,  of  Planaria,  153*,  154,  155. 
Pronuba  moth,  298. 
Prophase,  of  mitosis,  3o*-3i. 
Propolis,  256. 
Prosopyles,  145,  151*,  152. 
Prostomium,  165,  177*,  178. 
Proteid,  21,  22,  170. 
Prothorax,  236,  241*. 
Protohippus,  292. 
Protoplasm,   chemical   composition,    21- 

23;  bridges  of,  29,  97*;   properties  of, 

19-23  ;  specificity  of,  23  ;  structure  of, 

19-20*. 

Protopodite,  194*,  195,  197,  198, 199. 
Protozoa,  5,  7,  29,  31,  66,  80-99,  100,  101, 

112,  113,  228. 
Protozoaea,  230*,  231. 
Proventriculus,  169. 
Proximal,  117. 
Pseudopodiospore,  52,  112. 
Pseudopodium,  37*,  41,  80,  81,  124,  136, 

iSS- 

Ptyalin,  25. 
Pulvillus,  237*. 
Pupa,  252,  253*,  254. 
Pyloric  stomach,   195*,   200,   201. 
Pyrenoid,  83*,  85,  93. 

Queen  cell,  255. 

Queen  honeybee,  233,  234*,  249,  254,  255. 

Races,  of  bees,  261. 
Radial  canals,  145*,  147*,  152. 
Radial  symmetry,  4,  5,  7,  159. 
Radiating  canals,  of  Paramecium,  63. 
Rate:  of  regeneration,  190,  220-221;  of 

respiration,  244. 
Ray,  John,  269-270. 
Reactions,  16,  53-54  (see  also  Behavior), 


Recapitulation  theory,  228-232. 

Receptor,  179. 

Rectum,  161,  241*. 

Redi,  8. 

Reduction,  of  chromosomes,  103*,  104*, 
105*,  106,  107,  134. 

Reese,  117. 

Reflex,  178-179,  302-303. 

Regeneration:  Crayfish,  2i9*-22o; 
Earthworm,  i89*-i9o;  Grantia,  152; 
Hydra,  i37*-i39;  Planaria,  157*. 

Remak,  35. 

Reproduction,  15-16;  asexual,  15; 
sexual,  15-16:  Ameba,  50-53,  51*, 
112;  Ascaris,  161 ;  Chlamydomonas, 
92,  93;  Crayfish,  209-211;  Earth- 
worm, 180-185;  Eudorina,  96;  Eu- 
glena,  83*,  87;  Grantia,  148-149; 
Honeybee,  248-250;  Hydra,  114, 
118*,  I33*~i37;  Pandorina,  94,  95*, 
96,  113;  Paramecium,  67*— 71,  69*, 
112;  Planaria,  156;  Plasmodium,  89*, 
90-91,  113;  Spondylomorum,  94; 
Volvox,  96*,  97*~99,  114. 

Reproductive  organs,  159:  Ascaris, 
161*;  Crayfish,  209,  210*,  211*; 
Earthworm,  168,  180-185,  181*; 
Honeybee,  248,  249*,  250;  Hydra, 
1 1 8*,  119;  Planaria,  154*,  156. 

Reservoir,  of  Euglena,  83*,  84. 

Respiration,  14:  Ameba,  50;  Earthworm, 
175;  Grantia,  148;  Paramecium,  64, 
67. 

Respiratory  system:  Crayfish,  204; 
Honeybee,  243^244* . 

Reticular  theory  of  protoplasmic  struc- 
ture, 20*. 

Rheotropism,  54,  132. 

Rhizopoda,  80,  81,  82. 

Rhumbler,  48. 

Robin,  283-284,  294. 

Rocky  Mountain  sheep,  300. 

Ross,  88. 

Rostrum,  195*. 

Rotatoria,  5,  7. 

Round  worm,  5,  160-163,  298. 

Royal  jelly,  253. 

Ruthven,  A.  G.,  295. 

Sacculina  carcini,  299*-3OO. 
Salivary  glands,  241*,  242,  253. 
Saltation,  281. 


348 


INDEX 


Salts,  in  protoplasm,  21. 

Sapiens,  3. 

Saprophytic  nutrition,  85. 

Sarcode,  19. 

Scaphognathite,  196*,  197,  204. 

Scheel,  52. 

Schizogony,  89*,  90,  113. 

Schizont,  89*,  90. 

Schleiden,  34,  35. 

Schultze,  Max,  19,  35,  36,  271. 

Schwann,  34,  35,  271. 

Scleroblast,  146,  147*,  148. 

Scorpion,  6,  225,  228. 

Scyphozoa,  139. 

Sea  anemone,  139,  143*,  297. 

Sea  cucumber,  6. 

Sea  gull,  300. 

Sea  lily,  6. 

Sea  squirt,  6. 

Sea  urchin,  104. 

Sea  walnut,  5. 

Secretion,  46,  50. 

Segment :  Crayfish,  194*,  195 ;  Earth- 
worm, 165. 

Segmentation,  4,  6. 

Seminal  receptacle:  Crayfish,  212; 
Earthworm,  167,  180,  181*;  Honey- 
bee, 250,  251*. 

Seminal  vesicle:  Ascaris,  161 ;  Earth- 
worm, 181*,  182;  Honeybee,  248, 
249*;  Planaria,  154*,  156. 

Sense  organs :  Crayfish,  205-209 ;  Earth- 
worm, 180*;  Honeybee,  245-248. 

Senses,  of  Crayfish.  222. 

Sensitive  plant,  18. 

Sensory  neuron,  179*. 

Septum,  1 68*. 

Sertularia,  142. 

Seta,  1 65*-! 66*. 

Sexual  reproduction,  96,  97. 

Shell,  an  accretion,  12. 

Shrimp,  208. 

Siebolt,  von,  260. 

Sinuses,  of  crayfish,  203. 

Size,  of  organisms,  10. 

Skeleton,  of  sponges,  152. 

Sleeping  sickness,  82. 

Smell:  Crayfish,  222;  Honeybee,  247. 

Snail,  6,  287. 

Social  life,  297-302 ;  of  bees,  264-266. 

Somatic  cell,  97,  98,  100,  114,  115. 

Somatic  plasm,  102. 


Somatopleure,  186. 

Somite,  4,  6,  165. 

Spallanzani,  9. 

Special  creation,  8. 

Species,  1-3. 

Spermatheca:  Earthworm,  180,  181*, 
182;  Honeybee,  250,  251*. 

Spermatid,  103*,  104,  135,  184. 

Spermatocyst,  of  Grantia,  149. 

Spermatocyte,  68,  103* :  Earthworm, 
184;  Grantia,  149;  Honeybee,  250; 
Hydra,  134-135. 

Spermatogenesis,  102,  103*,  104 :  Cray- 
fish, 201;  Earthworm,  184;  Grantia, 
149;  Honeybee,  250;  Hydra,  134-135 

Spermatogonium,    103*,    134,    149, 
250. 

Spermatozoa,  15,  91,  99,  103*:   Crayf 
210*,     211 ;     Earthworm,    183,    184; 
Grantia,  149;   Honeybee,  250;  Hydr 
I35*>     Planaria,    156;     Plasmodii 
89*,  91,  113  ;  Volvox,  97*,  99. 

Spicule,  5,  144,  I47*-I48,  149,  152. 

Spider,  6,  225,  228. 

Spindle,  in  mitosis,  30*. 

Spinning  gland,  253*. 

Spiracle,  243*,  253*. 

Spiral  path,  of  Euglena,  87 ;  of  Pare 
cium,  65,  66. 

Spireme,  30*. 

Splanchnopleure,  186. 

Spondylomorum,  93*-94,  100. 

Sponge,  5,  i44*-i53- 

Spongitta,  152. 

Spongin,  152. 

Spontaneous  generation,  89. 

Spontaneous  movements,  73,  78,  128" 
129. 

Sporoblast,  89*,  91. 

Sporogany,  89*,  113. 

Sporozoa,  80,  82,  88,  89,  101. 

Sporozoite,  89*,  91,  113. 

Sport,  281. 

Sporulation,  15,  80,  112:  Ameba,  51,  S2« 

Spur,  236*,  237. 

Squirrel,  277. 

Starch,  17,  18. 

Starfish,  6. 

Statocyst,  197,  208,  222. 

Statolith,  208. 

Steapsin,  170. 

Sternal  artery,  195*,  203. 


INDEX 


349 


Sternal  sinus,  203. 

Sternum,  194*,  195,  238. 

Stigma,  83*,  84,  93. 

Stimuli,  1 6,  53,  101,  178;  external,  16, 
73,  78;  interference  of,  78*,  188;  in- 
ternal, 16;  localized,  130,  132;  non- 
localized,  130,  132. 

Sting,  239*,  254;  feeler,  239*. 

Stomach:  Crayfish,  195*,  200;  Honey- 
bee, 240,  241*. 

Stomatogastric  ganglion :  Crayfish,  205  ; 
Honeybee,  244,  245. 

Strainer,  201. 

Strasburger,  36. 

Striation,  on  cuticle  of  Earthworm,  167  : 
Euglena,  82,  83*;  Paramecium,  61*. 

Struggle  foE  existence,  283-284. 

Subintestinal  blood  vessel,  173. 

Submentum,  235. 

Subneural  blood  vessel,  172*,  173. 

Suboesophageal  ganglion,  205,  244. 

Subpharyngeal  ganglion,  177*. 

Supporting  tissue,  101*. 

Supraintestinal  blood  vessel,  173. 

Suprapharyngeal  ganglion,  177. 

Surface  tension,  Ameba,  42. 

Swarming,  Honeybee,  258. 

Swarm  spore,  95*,  96,  113. 

Swallow,  300. 

Swimmeret,  196*,  198,  199. 

Sycon,  149,  152. 

Symbiosis,  143*,  297-298. 

Symmetry,  4,  5,  6,  7,  158,  159. 

Sympathetic  nervous  system,  244,  245. 

Systematic  zoology,  i ;  historical,  269— 
270. 

Tapeworm,  158. 

Tapir,  279. 

Tarsus,  236*. 

Taste:    Crayfish,  222;    Honeybee,  247*. 

Telophase,  of  mitosis,  30*,  31. 

Telson,  195*. 

Tendon,  101. 

Tentacle,  Hydra,  116,  117,    118*,   120*, 

123-124,  125. 
Tergum,  194*,  238. 
Termite,  300. 
Testis:     Ascaris,    161 ;     Crayfish,    210*; 

Earthworm,  181*,  182;  Honeybee,  248, 

249*;    Hydra,    118*,    119,    134,    135*; 

Planaria,  154*,  156. 


Thermotropism,      53:     Ameba,     55-56; 

Hydra,   131 ;    Paramecium,  76-77,  78. 
Thigmotropism,     53:    Ameba,    54*~55*; 

Crayfish,  222-223 ;    Earthworm,  i£6- 

187;    Hydra,    130-131;    Paramecium, 

76*. 

Thorax,  236. 
Tibia,  236*. 
Tick,  6. 
Tissue,  epithelial,  100-101*;   connective, 

ioi*;    muscular,  101*;    nervous,  101- 

102;  supporting,  ioi*. 
Tongue,  235,  247*,  257. 
Touch:    Crayfish,  222;    Honeybee,  248 
Tower,  W.  L.,  295. 
Toxopneustes,  104. 
Trachea,  243*-244. 
Tracheata,  225. 
Trematoda,  158. 
Trembley,  133,  137. 
Trial   and   error,    in   behavior,   77,   86*p 

131- 

Trichina  spiralis,  163*. 
Trichinosis,  163. 
Trichocyst,  60,  62*,  63*,  81. 
Triploblastic,  4,  5,  6,  7,  no,  154,  159. 
Trochanter,  236*. 
Trophoplasm,  288. 
Tropism,  53. 

Trypanosoma  gambiense,  82. 
Trypsin,  170. 
Tunicata,  6,  7. 
Turbellaria,  158. 
Tyndall,  9. 

Typhlosolar  blood  vessel,  172*,  174. 
Typhlosole,  166*,  169. 

Undulating  membrane,  62,  66*,  67. 
Uniramous  appendage,  196*. 
Urea,  14,  50. 

Urinary  tubule,  241*,  242. 
Uropod,  199. 

Uterus:  Ascaris,  161*;  Planaria,  154* 
156. 

Vacuole,  contractile:  Ameba,  37*,  38,  39- 
41,  40*;  Chlamydomonas,  92*,  93; 
Euglena,  81,  82,  83*;  Paramecium, 
60,  63-64*,  67,  68;  Vohox,  97. 

Vagina:  Ascaris,  161*;  Honeybee,  249*, 
250;  Planaria,  154*,  156. 

Valve,  174,  175,  203,  242. 


350 


INDEX 


Variation,  continuous  or  fluctuating,  281 ; 
discontinuous,  281-282 ;  in  Parame- 
cium,  79. 

Vas  deferens:  Ascaris,  161;  Crayfish, 
210*;  Earthworm,  167,  181*,  182,  184; 
Honeybee,  248,  249*;  Planaria,  154*, 
156. 

Vascular  system :  Crayfish,  195*,  201- 
204;  Earthworm,  171,  172*;  Honey- 
bee, 241,*  242-244. 

Velum,  236*,  237. 

Ventral,  59;  abdominal  artery,  195*, 
200;  blood  vessel,  168,  172*,  173; 
nerve  cord,  177,  205,  241*;  thoracic 
artery,  195*,  203. 

Ventricle,  241*,  242. 

Vertebrata,  5,  6,  7. 

Vestigial  organs,  291. 

Virchow,  Rudolph,  10,  36,  271. 

Visceral  nervous  system,  205. 

Vision:  Crayfish,  207-208,  222;  Honey- 
bee, 246-247. 

Vitalism,  23,  24,  25. 

Vitelline  membrane,  252. 

Volvocacece,  81,  92,  96,  100. 

Volvocina,  81. 

Volvox,  81,  92,  97*-99,  loo,  no,  228, 
301. 

Vries,  Hugo  de,  274,  282,  295,  296. 

Walking:  Honeybee,  238. 
Wallace,  A.  R.,  301. 
Wandering  cell,  147,  157*. 
Wasp,  261,  264,  300. 


Water,  carried  by  bees,  256;  in  proto 
plasm,  21. 

Water  flea,  225. 

Wax,  254^  255 ;  glands,  239,  254 ;  pinch- 
ers, 236*,  238. 

Weismann,  A.,  274,  287,  288. 

Whale,  10,  277. 

Whitman,  C.  O.,  295,  296. 

Will,  304- 

Wilson,  E.  B.,  35. 

Wing:  of  Honeybee,  236,  238;  veins, 
238. 

Wolff,  F.  K.,  271-272. 

Wolves,  301. 

Worker  honeybee,  234* ;  cell,  255*. 

Worms,  flat,  5 ;  jointed,  6 ;  round,  5. 

Yellow  fever  parasite,  80. 

Yerkes,  R.  M.,  224. 

Yolk,  glands,  154*,  156;  pyramids,  215*; 

spheres,  214,  251. 
Youth,  14,  71,  72. 
Yucca,  pollination  by  moth,  298. 

Zoaea,  231*,  232. 

Zoochlorella,  143,  297. 

Zoogeography,  2,  275-279. 

Zooid,  139. 

Zoology,  distributional,   2 ;  evolutionary, 

2;  historical,  267-274;  of  to-day,  274; 

systematic,  i. 
Zoophyte,  139. 
Zygote,  91,  93,  94,  95*,  96,  98,  107,  113. 

114,  us- 


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